A few refactors of lint.py

[thejcannon]

The big changes are:

• Bumping the "metadata" of a partition to be a first-class-citizen and renaming it to "PartitionKey" to better encompass its intent.
• Making Partitions a mapping from key -> partition (with the key option for single partitions. Will be None)
• Refactored the memoized class property types to be unconditionally declared with most of the relevant type-checking-and-runtime info, and only conditionally declare the runtime version (which inherits from the both-type)

[ci skip-build-sheels]
[ci skip-rust]

[thejcannon]

These are required so we can use them as the "base" class. Otherwise we'd hit an infinite recursion trying to lookup the class.

[stuhood]

Unrelated to this one in particular, but: it looks like convert_fmt_result_to_lint_result is currently marked level=INFO:

10:58:39.61 [INFO] Completed: Format with docformatter - docformatter made no changes.
10:58:39.61 [INFO] Completed: pants.core.goals.lint.convert_fmt_result_to_lint_result - docformatter succeeded.
10:58:39.88 [INFO] Completed: Format with isort - isort made no changes.
10:58:39.89 [INFO] Completed: pants.core.goals.lint.convert_fmt_result_to_lint_result - isort succeeded.
10:58:40.36 [INFO] Completed: Format with Autoflake - autoflake made no changes.
10:58:40.36 [INFO] Completed: pants.core.goals.lint.convert_fmt_result_to_lint_result - autoflake succeeded.
10:58:41.49 [INFO] Completed: Format with Black - black made no changes.
10:58:41.49 [INFO] Completed: pants.core.goals.lint.convert_fmt_result_to_lint_result - black succeeded.

[thejcannon]

It has taken me until today to actually realize why level=LogLevel.DEBUG is used in rule(). LOL.

I'll fix it in the fmt big refactor, as I'm moving the rule in that branch anyways,