bpo-43926: Cleaner metadata with PEP 566 JSON support.

[jaraco]

https://bugs.python.org/issue43926

[jaraco]

Note, this change includes and should be merged after #25297.

[tirkarthi]

This is a Python 3.10 only feature and hence a versionadded directive can be added.

Slightly out of topic, are Distribution objects and metadata attribute immutable? Accessing json attribute from metadata objects repeats the computation though it looks like a computed once attribute so just curious if metadata attribute of Distribution, json attribute etc. can be made a cached_property.

[jaraco]

Fixed in 289665029e (also versionchanged for changes to the interpretation of the metadata).

[jaraco]

can be made a cached_property?

Although technically the PackageMetadata object is mutable, the protocol for it doesn't allow for mutation, so it's effectively immutable, so yes, a cached_property could be employed. Such an optimization may be premature, so I'm slightly inclined to defer that concern until a use-case presents itself. I don't feel strongly about it though and would likely accept a PR to importlib_metadata.

[jaraco]

@warsaw I note also that while we intentionally consolidated importlib.metadata into a single module (#15153), the package has since diversified and the _common module now conflicts with importlib_resources, so I've moved the modules back into a package to avoid the conflict and more clearly separate the concerns.

[jaraco]

In order to ensure this change is included in the 3.10 release, I'm merging now. Happy to troubleshoot or back out specific behaviors as needed.