Store license-files in licenses subfolder

[cdce8p]

Summary of changes

The initial draft of PEP 639 specified that license files should be written directly to the .dist-info folder. This was changed in later iterations. The files should now be written to .dist-info/licenses and the root folder structure should be recreated to avoid name conflicts.

Additional changes to be spec compliant:

• Added support for recursive license_files specifier, e.g. **/LICENSE.
• Normalize License-File path delimiter.

Closes #3596
Refs #4629

[abravalheri]

Should we use os.sep here instead of \\ (e.g. if somehow \\ ends showing up on a linux build, we want the build to crash right, because it would mean incorrect input from the user)?

[cdce8p]

I don't believe that's necessary here. path isn't a user input but rather the glob matched license path from self._expand_patterns. The idea with this line is just to normalize the separator since the core metadata field only allows forward /.

[abravalheri]

How about valid paths in linux containing \? (e.g. mkdir 'hello\world' && touch 'hello\world/LICENSE.txt')... It is a bit of a problematic choice if the user decides to do that, but os.sep would be less error prone in that case, right?

[cdce8p]

Not sure I've ever seen that in the wild but you're right it's a possibility. Pushed 3e9b9c7 to use os.sep instead.

[abravalheri]

Thank you!