[match-case] fix matching against typing.Callable and Protocol types.

[randolf-scholz]

• Fixes match-case class checking against collection.abc.Callable raises error [misc] #14014
• Partially addresses [match-case]/[isinstance] narrowing logic against the special AnyCallable Protocol differs from callable() check #19470

Added extra logic in checker.py:conditional_types function to deal with structural types such as typing.Callable or protocols.

new tests

• testMatchClassPatternCallable: tests case Callable() as fn usage
• testMatchClassPatternProtocol: tests case Proto() usage, where Proto is a Protocol
• testMatchClassPatternCallbackProtocol: tests case Proto() usage, where Proto is a Callback-Protocol
• testGenericAliasIsinstanceUnreachable: derived from a mypy-primer failure in mesonbuild. Tests that isinstance(x, Proto) can produce unreachable error.
• testGenericAliasRedundantExprCompoundIfExpr: derived from a CI failure of python runtest.py self of an earlier version of this PR.

modified tests

• testOverloadOnProtocol added annotations to overload implementation, which wasn't getting checked. Added missing return. Fixed return type in second branch.

[sterliakov]

Also, does this fix #19470 completely or partially? Please update the description to either remove "partially" or remove "fixes" magic word, so that half-resolved issue does not get autoclosed

[sterliakov]

Thanks! I like your docstring suggestion in the last comment, could you merge it?

[randolf-scholz]

@sterliakov done.

[ilevkivskyi]

OK, looks like this is something worth trying

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/http.py:607: error: Unused "type: ignore" comment  [unused-ignore]