Maven Enforcer Rule itself is prone to dependency conflicts of aether library

I tried to apply the linkage checker enforcer rule to the project when troubleshooting https://github.com/googleapis/java-spanner-jdbc/pull/251#issuecomment-720702035, however the linkage checker failed:

```
[INFO] --- maven-enforcer-plugin:3.0.0-M3:enforce (enforce-linkage-checker) @ google-cloud-spanner-jdbc ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 27.293 s
[INFO] Finished at: 2020-11-03T11:00:01-05:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce (enforce-linkage-checker) on project google-cloud-spanner-jdbc: Execution enforce-linkage-checker of goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce failed: An API incompatibility was encountered while executing org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce: java.lang.IllegalAccessError: tried to access method org.eclipse.aether.util.ChecksumUtils.toHexString([B)Ljava/lang/String; from class org.eclipse.aether.connector.basic.ChecksumCalculator$Checksum
[ERROR] -----------------------------------------------------
[ERROR] realm = plugin>org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3
[ERROR] strategy = org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy
[ERROR] urls[0] = file:/Users/suztomo/.m2/repository/org/apache/maven/plugins/maven-enforcer-plugin/3.0.0-M3/maven-enforcer-plugin-3.0.0-M3.jar
[ERROR] urls[1] = file:/Users/suztomo/.m2/repository/com/google/cloud/tools/linkage-checker-enforcer-rules/1.5.4/linkage-checker-enforcer-rules-1.5.4.jar
[ERROR] urls[2] = file:/Users/suztomo/.m2/repository/com/google/cloud/tools/dependencies/1.5.4/dependencies-1.5.4.jar
[ERROR] urls[3] = file:/Users/suztomo/.m2/repository/org/apache/bcel/bcel/6.5.0/bcel-6.5.0.jar
[ERROR] urls[4] = file:/Users/suztomo/.m2/repository/org/apache/maven/resolver/maven-resolver-transport-http/1.4.2/maven-resolver-transport-http-1.4.2.jar
[ERROR] urls[5] = file:/Users/suztomo/.m2/repository/org/apache/httpcomponents/httpcore/4.4.10/httpcore-4.4.10.jar
[ERROR] urls[6] = file:/Users/suztomo/.m2/repository/org/apache/maven/resolver/maven-resolver-transport-file/1.4.2/maven-resolver-transport-file-1.4.2.jar
[ERROR] urls[7] = file:/Users/suztomo/.m2/repository/org/apache/maven/resolver/maven-resolver-connector-basic/1.4.2/maven-resolver-connector-basic-1.4.2.jar
[ERROR] urls[8] = file:/Users/suztomo/.m2/repository/org/apache/httpcomponents/httpclient/4.5.12/httpclient-4.5.12.jar
[ERROR] urls[9] = file:/Users/suztomo/.m2/repository/commons-logging/commons-logging/1.2/commons-logging-1.2.jar
[ERROR] urls[10] = file:/Users/suztomo/.m2/repository/org/relaxng/jing/20181222/jing-20181222.jar
[ERROR] urls[11] = file:/Users/suztomo/.m2/repository/net/sf/saxon/Saxon-HE/9.6.0-4/Saxon-HE-9.6.0-4.jar
[ERROR] urls[12] = file:/Users/suztomo/.m2/repository/xerces/xercesImpl/2.9.1/xercesImpl-2.9.1.jar
[ERROR] urls[13] = file:/Users/suztomo/.m2/repository/xml-apis/xml-apis/1.0.b2/xml-apis-1.0.b2.jar
[ERROR] urls[14] = file:/Users/suztomo/.m2/repository/isorelax/isorelax/20030108/isorelax-20030108.jar
[ERROR] urls[15] = file:/Users/suztomo/.m2/repository/commons-cli/commons-cli/1.4/commons-cli-1.4.jar
[ERROR] urls[16] = file:/Users/suztomo/.m2/repository/com/google/guava/guava/29.0-jre/guava-29.0-jre.jar
[ERROR] urls[17] = file:/Users/suztomo/.m2/repository/com/google/guava/failureaccess/1.0.1/failureaccess-1.0.1.jar
[ERROR] urls[18] = file:/Users/suztomo/.m2/repository/com/google/guava/listenablefuture/9999.0-empty-to-avoid-conflict-with-guava/listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar
[ERROR] urls[19] = file:/Users/suztomo/.m2/repository/com/google/code/findbugs/jsr305/3.0.2/jsr305-3.0.2.jar
[ERROR] urls[20] = file:/Users/suztomo/.m2/repository/org/checkerframework/checker-qual/2.11.1/checker-qual-2.11.1.jar
[ERROR] urls[21] = file:/Users/suztomo/.m2/repository/com/google/errorprone/error_prone_annotations/2.3.4/error_prone_annotations-2.3.4.jar
[ERROR] urls[22] = file:/Users/suztomo/.m2/repository/com/google/j2objc/j2objc-annotations/1.3/j2objc-annotations-1.3.jar
[ERROR] urls[23] = file:/Users/suztomo/.m2/repository/org/codehaus/mojo/extra-enforcer-rules/1.3/extra-enforcer-rules-1.3.jar
[ERROR] urls[24] = file:/Users/suztomo/.m2/repository/org/apache/maven/shared/maven-dependency-tree/2.2/maven-dependency-tree-2.2.jar
[ERROR] urls[25] = file:/Users/suztomo/.m2/repository/org/eclipse/aether/aether-util/0.9.0.M2/aether-util-0.9.0.M2.jar
[ERROR] urls[26] = file:/Users/suztomo/.m2/repository/junit/junit/4.11/junit-4.11.jar
[ERROR] urls[27] = file:/Users/suztomo/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar
[ERROR] urls[28] = file:/Users/suztomo/.m2/repository/commons-codec/commons-codec/1.12/commons-codec-1.12.jar
[ERROR] urls[29] = file:/Users/suztomo/.m2/repository/org/apache/maven/shared/maven-common-artifact-filters/3.1.0/maven-common-artifact-filters-3.1.0.jar
[ERROR] urls[30] = file:/Users/suztomo/.m2/repository/org/apache/maven/shared/maven-shared-utils/3.1.0/maven-shared-utils-3.1.0.jar
[ERROR] urls[31] = file:/Users/suztomo/.m2/repository/commons-io/commons-io/2.5/commons-io-2.5.jar
[ERROR] urls[32] = file:/Users/suztomo/.m2/repository/org/sonatype/sisu/sisu-inject-bean/1.4.2/sisu-inject-bean-1.4.2.jar
[ERROR] urls[33] = file:/Users/suztomo/.m2/repository/org/sonatype/sisu/sisu-guice/2.1.7/sisu-guice-2.1.7-noaop.jar
[ERROR] urls[34] = file:/Users/suztomo/.m2/repository/org/sonatype/aether/aether-util/1.7/aether-util-1.7.jar
[ERROR] urls[35] = file:/Users/suztomo/.m2/repository/org/codehaus/plexus/plexus-interpolation/1.14/plexus-interpolation-1.14.jar
[ERROR] urls[36] = file:/Users/suztomo/.m2/repository/org/codehaus/plexus/plexus-component-annotations/1.7.1/plexus-component-annotations-1.7.1.jar
[ERROR] urls[37] = file:/Users/suztomo/.m2/repository/org/sonatype/plexus/plexus-sec-dispatcher/1.3/plexus-sec-dispatcher-1.3.jar
[ERROR] urls[38] = file:/Users/suztomo/.m2/repository/org/sonatype/plexus/plexus-cipher/1.4/plexus-cipher-1.4.jar
[ERROR] urls[39] = file:/Users/suztomo/.m2/repository/org/codehaus/plexus/plexus-utils/3.3.0/plexus-utils-3.3.0.jar
[ERROR] urls[40] = file:/Users/suztomo/.m2/repository/org/apache/maven/enforcer/enforcer-api/3.0.0-M3/enforcer-api-3.0.0-M3.jar
[ERROR] urls[41] = file:/Users/suztomo/.m2/repository/org/apache/maven/enforcer/enforcer-rules/3.0.0-M3/enforcer-rules-3.0.0-M3.jar
[ERROR] urls[42] = file:/Users/suztomo/.m2/repository/org/apache/commons/commons-lang3/3.8.1/commons-lang3-3.8.1.jar
[ERROR] urls[43] = file:/Users/suztomo/.m2/repository/org/beanshell/bsh/2.0b4/bsh-2.0b4.jar
[ERROR] urls[44] = file:/Users/suztomo/.m2/repository/org/apache/maven/resolver/maven-resolver-util/1.4.1/maven-resolver-util-1.4.1.jar
[ERROR] Number of foreign imports: 1
[ERROR] import: Entry[import from realm ClassRealm[project>com.google.cloud:google-cloud-spanner-jdbc:1.17.3-SNAPSHOT, parent: ClassRealm[maven.api, parent: null]]]
[ERROR]
[ERROR] -----------------------------------------------------
```

Probably the same cause as https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/1091, but as it happened twice, we should better to document or show message for such situation.