Reorganized and reworked integration tests

[lefou]

The integration tests are now organized as follows:

integration/            # integration tests for mill
├── forked              # in a forked process (more realistic)
├── local               # from the running process (faster)
└── thirdparty          # tests building some thirdparty projects with mill
    ├── forked          # in a forked process
    └── local           # from the running process (faster)

Forked tests now no longer require a mill binary installed outside of mill. Instead, some new targets internally create a deployment and use this for the tests.

Run these tests with:

mill integration.local
mill integration.forked
mill integration.thirdparty.local
mill integration.thirdparty.forked
mill integration.__.forked                                            # run all forked tests
mill integration.local.test mill.integration.local.LargeProjectTests  # run selective suite

Since zinc 1.6.0, compilation with the specific scala 2.12.4 version is almost unusable. Unfortunately, the version of PlayJson we previously build in the thirdparty integration tests no longer works. I tried to adapt it for older or newer versions but in the end, completely removed this project and repo.

If anybody wants to contribute other projects, e.g. some Scala 3 projects, to our integration test suite, then please go ahead and open a pull request.