add scala_platform to export output for metals!

[cosmicexplorer]

Problem

We would like to support the Metals VSCode plugin -- see associated PR at scalameta/metals#935. Pants currently does not have any fields which specifically provide the scala version and the compiler jars, so that PR currently iterates over the libraries keys, which doesn't provide all the scala compiler jars.

Solution

Add a scala_platform key to the ./pants export output containing the scala_version and compiler_classpath:

{
  ...,
  "scala_platform": {
    "scala_version": "2.12",
    "compiler_classpath": [
      "/path/to/scala-compiler-2.12.8.jar",
      "/path/to/scala-library-2.12.8.jar",
      ...
    ]
}

Result

Using the branch at https://github.com/cosmicexplorer/language-server/tree/sohamr/add-pants-build-tool (which is based off of scalameta/metals#935), I have been able to show that metals can extract the scala_platform from the json!

[olafurpg]

This is great! Thank you. I’ll update the Metals PR to use the new Scala platform key if it exists with a fallback to the current workaround.

[wisechengyi]

Thanks, @cosmicexplorer ! looks good. One thing we need to bump the export version from
1.0.10 to 1.0.11:

pants/src/python/pants/backend/project_info/tasks/export.py

Line 58 in 51aca06

DEFAULT_EXPORT_VERSION = '1.0.10'

pants/tests/python/pants_test/backend/project_info/tasks/test_export.py

Line 252 in 23a8a2b

self.assertEqual('1.0.10', result['version'])

[wisechengyi]

Sorry one more: update the doc: https://github.com/pantsbuild/pants/blob/ecd15542eefff1d112a5ce96ac4ec3f871dbd21a/src/docs/export.md#export-format-changes

[cosmicexplorer]

@wisechengyi bumped the export format version and added docs, and also added testing! It required adding a lot of new targets in order to allow the export task to pull the scala platform jars, which will always be injected in a normal pants build, but not in v1 task tests such as this.