Bump to IntelliJ 2020.3

[tpasternak]

Commit 1

Switch host pants to a custom one with updated Zinc

The pants version we've been using was using an old Zinc version that did not support Scala 2.13. Since this commit, we build by ourselves pants with bumped Zinc version, so upgrade to 2.13 is possible.

Commit 2

Bump to IntelliJ 2020.3

• Scala Plugin 2020.3 is based on Scala 2.13, so we need to bump it, too
• isBspProject required reimplementation (same as in scala plugin)
• OpenProjectTask constructor signature has changed in 2020.3 (new argument)
• Module setOption/getOption methods are deprecated, so it was required to module.setOption(EXTERNAL_SYSTEM_ID_KEY, ...) manually. IJ Core not doing it any more
• allowRootAccess doesn't allow / argument any more
• Test Fixtures sometimes add MockSDKs so it was needed to clean it manually
• 2.13 contains new collection converters - we needed to adapt

[tpasternak]

@wisechengyi could you look at cd446a1 if it's acceptable?

[callicles]

What's missing for this to get merged? Maybe I can give a hand?

[tpasternak]

Hey @callicles sorry for the late reply. I'm now trying to find some other way to build testData/external-system-test-api.jar than just local compilation on a laptop. After it is done, I think the PR will be ready to review