Explicit handling of scalac plugins and related enablement options

[lefou]

We no longer generate an enablement option for each transitively resolved plugin jar, but instead, only generated one -Xplugin: option for each explicitly given plugin. This will fix previously seen warnings for the scalac compiler.

I took the opportunity to clean things up on the go.
I removed the handling of -Xplugin: from the ZincWorker into the ScalaModule.
This fixes the issue that both the BSP server implementation and the Bloop Config exporter needed to handle this on their own.
Now, this is handled in the IMHO right place, is properly shared and still overrideable by the user if the situation calls for it.

I recognized, that we don't support scalac plugin dependencies from in-project modules at the moment, but this can be added later if needed. Previously, it was enough to add the module.jar() to the scalacPluginClassapth, now it is also required to add the s"-Xplugin:{module.jar().path}" manually. Not to hard, but a future scalacPluginModuleDeps target could make this easier. As this also hits Mills build when we bootstrap our own project, that's why I did that change to build.sc.

Fix #1690
Fix #1691

[lolgab]

Left a comment about visibility and naming

[lolgab]

Github wants me to add a comment otherwise I can't send inline comments.

[lolgab]

Also docJar should use the new target:

mill/scalalib/src/ScalaModule.scala

Line 236 in 1af99f5

scalaDocPluginClasspath().map(pluginPathRef => s"-Xplugin:${pluginPathRef.path}")

[lolgab]

You know you're doing something right, when to fix a bug you also simplify code. Now everything makes more sense and bloop config and other take directly the Mill output without any modification. Congrats!
I left a little comment about a name.
Great the new targets are private so we don't have to commit on the naming/existence.

[ckipp01]

Nice! From trying it out the errors you normally see in the Metals logs about the unfound scalac-plugin.xml files are gone:

No More 🚀

2022.01.22 15:54:11 ERROR /Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.0.6/scala-xml_2.12-1.0.6.jar
java.nio.file.NoSuchFileException: /scalac-plugin.xml

And as I assumed because of the above, the Bloop export looks correct as well:

        "scala": {
            "organization": "org.scala-lang",
            "name": "scala-compiler",
            "version": "2.12.15",
            "options": [
                "-Xplugin:/Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scalamacros/paradise_2.12.15/2.1.1/paradise_2.12.15-2.1.1.jar",
                "-Ywarn-unused"
            ],
            "jars": [
                "/Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-compiler/2.12.15/scala-compiler-2.12.15.jar",
                "/Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-reflect/2.12.15/scala-reflect-2.12.15.jar",
                "/Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.12.15/scala-library-2.12.15.jar",
                "/Users/ckipp/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.0.6/scala-xml_2.12-1.0.6.jar"
            ]
        },