WIP: Fix build functionality post auto-migration

[ajaychandran]

Follow up to #5428 that adds several enhancements and fixes to enable build functionality.

• Set mill-jvm-version to system.
• Added support for converting BOM modules.
• Added mappings for depManagement, sourcesFolders, sources, resources, test forkArgs.
• Added workaround for running Junit5 tests.
• Added mapping for Gradle dependency version range.

Summary of code changes

• libs/init/package.mill: Added mock tests to check build functionality post migration. Migrated modules to Scala 3.8.0-RC2; this bumps the minimum JDK requirement to version 17.
• libs.init.buildgen.api: Added ModuleSpec.Value and ModuleSpec.Values to capture cross-version specific values and inheritance relationships for module members defined in ModuleSpec.
• libs.init.buildgen: Added BuildGen for generating meta-build types and writing build files using ModuleSpec.
• libs.init.buildgen.test: Updated BuildGenChecker to test implementations with a custom JAVA_HOME.
• libs.init.gradle.exportplugin: Updated BuildModelBuilder.toPackage and added enhancements/fixes.
• libs.init.gradle: Refactored and updated GradleBuildGenMain.
• libs.init.gradle.test: Removed tests for Gradle version 6.0 and 7.0 and added test for version 7.2 (minimum version that runs on JDK 17).
• libs.init.maven: Added workaround to retain BOM dependencies and fixed configuring system properties in Modeler. Refactored Plugins and fixed errorProneMvnDeps derivation. Refactored and updated MavenBuildGenMain.
• libs.init.sbt.exportplugin: Updated ExportBuildPlugin.exportProject and added enhancements/fixes.
• libs.init.sbt: Refactored and updated SbtBuildGenMain.

Pending items

• Identify further enhancements and expand mock test suite.
• Address related open issues.
  • ./mill init on lichess-org/lila results in invalid test suite modules #5536
  • Add heuristics to translate common tools and plugins during ./mill init from SBT, Maven, and Gradle #5571
  • Failing tests in integration.manual[migrating] module #6039
  • mill init should provide a --help option #6043
  • Crash when importing gradle projects ghidra or RxJava #6075
  • Importing jadx gradle project fails #6076
  • Make ./mill init work for spring-boot-examples project #6077
  • ./mill init on the Netty project fails compilation #6078
  • ./mill init fails to compile for Mockito codebase #6080
  • ./mill init fails for TheAlgorithms/Java and alibaba/fastjson2 #6081
• Update integration tests.
• Update user documentation.

[lihaoyi]

@ajaychandran if the heuristics to guess the java version from the compiler flags don't work well in practice, maybe we should just go with mill-java-version: system then. Trying to look at the user JAVA_HOME and expecting the user to set it to the right place is probably too error prone for new Mill users to do reliably

[ajaychandran]

@ajaychandran if the heuristics to guess the java version from the compiler flags don't work well in practice, maybe we should just go with mill-java-version: system then. Trying to look at the user JAVA_HOME and expecting the user to set it to the right place is probably too error prone for new Mill users to do reliably

We auto-detect the JVM version from the system JDK in ModuleConfig.JavaHomeModule.system and set a concrete version for jvmId. The user is not required to set a version anywhere.
Only the environment needs to be set up with the correct JAVA_HOME. This should already be true on a development system.

Using mill-java-version: system will also work. The loss of build reproducibility is acceptable I suppose.

[lihaoyi]

Got it. If we want to auto-detect the JVM version, how about just shelling out to java -version? That should give us a version to use and would be simpler than the logic you currently have in lazy val system

[ajaychandran]

On second thoughts, I will make the switch to the mill-java-version: system option.
This will minimize changes to any CI workflows and is semantically more accurate.

[ajaychandran]

@lihaoyi The PR description has been updated with the summary of code changes.