Investigate Gradle Tooling API support

The current solution to find the dependencies of a module/project invokes Gradle from the command line and parses the output:

https://github.com/fwcd/KotlinLanguageServer/blob/322594f491369f51fdda33295101098d81d5bf3d/src/main/kotlin/org/javacs/kt/classpath/GradleDependencyResolver.kt#L54-L62

A more elegant solution, however, would be to have direct, programmatic access to Gradle's APIs in order to fetch a list of dependency JARs. Both Eclipse and IntelliJ IDEA have custom project models inside the Gradle API that fit their specific use cases, but which can be used externally too (`EclipseModel` and `IdeaModel`). Unfortunately, these do have some limitations regarding Android support, which is why the current solution involves a [custom Gradle script](https://github.com/fwcd/KotlinLanguageServer/blob/master/src/main/resources/classpathFinder.gradle) that is invoked as described above. Instead, a custom model could be built using the Tooling API.

Resources:
* https://github.com/gradle/gradle/issues/4215#issuecomment-361840825
* https://docs.gradle.org/current/javadoc/org/gradle/tooling/provider/model/ToolingModelBuilderRegistry.html (Tooling API extensibility points)
* https://stackoverflow.com/questions/40739674/external-registration-of-tooling-api-custom-models
* https://github.com/gradle/gradle/blob/38e0df22ecff6c894f5c3eeecb94106e2ba557ff/subprojects/docs/src/samples/toolingApi/eclipse/src/main/java/org/gradle/sample/Main.java#L28-L28 (EclipseModel sample)
* https://github.com/gradle/gradle/blob/38e0df22ecff6c894f5c3eeecb94106e2ba557ff/subprojects/docs/src/samples/toolingApi/idea/src/main/java/org/gradle/sample/Main.java (IdeaModel sample)
* https://mvnrepository.com/artifact/org.gradle/gradle-kotlin-dsl-tooling-models/0.18.1 (DSL tooling models)