[swagger-codegen-maven-plugin] Unable to process multiple inputSpecs

Description

Using version 3 of the Swagger Codegen Maven plugin I am unable to run multiple executions each using a different inputSpec. When I do this I get the following error:

[INFO] --- swagger-codegen-maven-plugin:3.0.13:generate (default) @ my-project ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  13.445 s
[INFO] Finished at: 2020-01-02T20:24:38-05:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal io.swagger.codegen.v3:swagger-codegen-maven-plugin:3.0.13:generate (default) on project my-project: The parameters 'inputSpec', 'language' for goal io.swagger.codegen.v3:swagger-codegen-maven-plugin:3.0.13:generate are missing or invalid -> [Help 1]

For both executions I have an ID specified and its own configuration as follows:

<plugin>
	<groupId>io.swagger.codegen.v3</groupId>
	<artifactId>swagger-codegen-maven-plugin</artifactId>
	<executions>
		<execution>
			<id>content-push</id>

			<configuration>
				<inputSpec>${project.basedir}/src/main/swagger/contentPush/swagger.yaml</inputSpec>
				<language>java</language>
				<modelPackage>${content.push.default.client.package}.model</modelPackage>
				<apiPackage>${content.push.default.client.package}.api</apiPackage>
				<invokerPackage>${content.push.default.client.package}</invokerPackage>
				<modelNamePrefix>ContentPush</modelNamePrefix>
				<configOptions>
					<dateLibrary>java8</dateLibrary>
				</configOptions>
			</configuration>
		</execution>
		<execution>
			<id>distribution</id>

			<configuration>
				<inputSpec>${project.basedir}../cpr/xxx-cpr-distribution/src/main/swagger/distribution/swagger.yaml</inputSpec>
				<language>java</language>
				<modelPackage>${distribution.client.package}.model</modelPackage>
				<apiPackage>${distribution.client.package}.api</apiPackage>
				<invokerPackage>${distribution.client.package}</invokerPackage>
				<modelNamePrefix>Dist</modelNamePrefix>
				<configOptions>
					<dateLibrary>java8</dateLibrary>
				</configOptions>
			</configuration>
		</execution>
	</executions>
</plugin>

If I run either one of them without the ID it works, however if I run just one with the ID specified I get the same error. This leads me to believe the issue is more with the ID being specified, however if I leave out the ID I get the following error:

[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[WARNING] 'dependencies.dependency.version' for org.junit.jupiter:junit-jupiter:jar is either LATEST or RELEASE (both of them are being deprecated) @ xxx:my-project:[unknown-version], location\to\the\pom.xml, line 80, column 22
[ERROR] 'build.plugins.plugin[io.swagger.codegen.v3:swagger-codegen-maven-plugin].executions.execution.id' must be unique but found duplicate execution with id default @ xxx:my-project:[unknown-version], location\to\the\pom.xml, line 108, column 32
 @ 
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]   
[ERROR]   The project xxx:my-project:1.0.49-SNAPSHOT (location\to\the\pom.xml) has 1 error
[ERROR]     'build.plugins.plugin[io.swagger.codegen.v3:swagger-codegen-maven-plugin].executions.execution.id' must be unique but found duplicate execution with id default @ xxx:my-project:[unknown-version], location\to\the\pom.xml, line 108, column 32

Swagger-codegen version

3.0.13 and 3.0.14

Swagger declaration file content or url

N/A

Command line used for generation

N/A

Steps to reproduce

This assumes two valid OpenAPI YAML or JSON files.

1. Create a Maven project that will be used to generate the source.
2. Add the io.swagger.codegen.v3:swagger-codegen-maven-plugin plugin with one execution to validate everything works.
3. Add the second execution and populate and ID for both executions.

Related issues/PRs

Suggest a fix/enhancement

Start by reproducing the Maven configuration with two executions of swagger-codegen-maven-plugin, each with its own ID, inputSpec, and language. Then inspect the plugin's execution-parameter handling. Done means both executions retain their individual configuration and the Maven build completes successfully without duplicate-ID or missing-parameter errors.