[WIP] multi-input Get selectors

[tdyas]

WIP take on the syntax parts of #7490

[tdyas]

Unclear to me what the types here should even be.

I am getting this error when even trying to run fmt on this code:

Traceback (most recent call last):
  File "/Users/tdyas/TC/pants/src/python/pants/bin/pants_loader.py", line 19, in <module>
    from pants.bin.pants_runner import PantsRunner
  File "/Users/tdyas/TC/pants/src/python/pants/bin/pants_runner.py", line 13, in <module>
    from pants.bin.remote_pants_runner import RemotePantsRunner
  File "/Users/tdyas/TC/pants/src/python/pants/bin/remote_pants_runner.py", line 15, in <module>
    from pants.option.global_options import GlobalOptions
  File "/Users/tdyas/TC/pants/src/python/pants/option/global_options.py", line 27, in <module>
    from pants.engine.environment import CompleteEnvironment
  File "/Users/tdyas/TC/pants/src/python/pants/engine/environment.py", line 10, in <module>
    from pants.engine.rules import collect_rules, rule
  File "/Users/tdyas/TC/pants/src/python/pants/engine/rules.py", line 29, in <module>
    from pants.engine.goal import Goal
  File "/Users/tdyas/TC/pants/src/python/pants/engine/goal.py", line 12, in <module>
    from pants.option.subsystem import Subsystem
  File "/Users/tdyas/TC/pants/src/python/pants/option/subsystem.py", line 12, in <module>
    from pants.engine.internals.selectors import AwaitableConstraints, Get
  File "/Users/tdyas/TC/pants/src/python/pants/engine/internals/selectors.py", line 323, in <module>
    class Effect(Generic[_Output, _Input], Awaitable[_Output, _Input]):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

cc @Eric-Arellano

[stuhood]

Ah, so: this aspect of the description was stale over there. We actually only parse Get(C, B, b), etc: while the indexing for type hints is supported, it's usually inferred, and the actual input type is required currently.

We briefly did actually support index-only (Get[C](B, b)), but we removed it. Sorry for the confusion.

[tdyas]

Is there still an intent to have the other forms that use brackets though?

That is:

await Get[E](Params((C, c), (D, d))) 
await Get[F](Params(C(...), D(...)))

[tdyas]

The change would still need to have use of brackets to return a trampoline which isn't required if the syntax does not use brackets at all.