Drop MethodOrHKCommon, ensure HigherKinded is properly labeled

[Blaisorblade]

No description provided.

[Blaisorblade]

EDIT: ignore this message, it's about an unrelated spurious failure.

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This failed locally but not on CI, I suppose it's because I didn't clean before launching... but would cleaning hide some bug?
EDIT: investigating.
EDIT2: IIUC, the test fails/is flaky on a clean compile on case-insensitive filesystems, because overloaded_ho_fun.scala triggers a case-sensitivity warning:

-- Warning: /Users/pgiarrusso/git/dotty/tests/pos/overloaded_ho_fun.scala:89:6 -
89 |trait SAM[-T, +R] { def apply(x: T): R }
   |      ^
   |Class SAM differs only in case from Sam. Such classes will overwrite one another on case-insensitive filesystems.
one warning found

so on case-insensitive FSs that test can only compile, not actually work, and not compile from TASTY IIUC.

Indeed, the new run gave a different error:

[info] Test dotty.tools.dotc.FromTastyTests.posTestFromTasty started
[=======================================] completed (1466/1466, 70s)
undefined: B2.t.f # 6950539: TermRef(TermRef(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),module class B2$)),module t),f) at frontend
[=======================================] completed (1362/1362, 39s)

Yet, that file passes on testCompilation despite the warning (confirmed with testCompilation overloaded_ho_fun). I'll fix the test in a separate PR, but I got questions on the infrastructure.

[liufengyun]

It seems it's just a refactoring. LGTM. Maybe @odersky should have a look.

[Blaisorblade]

Opened #4258 for the local issues.