Pip: add a command to use pip's build isolation for building a sdist

[RonnyPfannschmidt]

i believe it would be a nice help for the python ecosystem to be able to build a sdist using the same build isolation as pip is using to turn sdists or source trees into a wheel

[brettcannon]

Just came across a desire for a pip sdist equivalent to pip wheel as well to help future-proof a CI pipeline to create everything someone would want to upload to PyPI.

[pradyunsg]

There's #5407 that discusses a pip build command which would satisfy this need.

[chrahunt]

This use case is considered in specifically in #6041, which was branched from the above.

[pradyunsg]

Thanks for that link @chrahunt! I'd completely forgotten about this issue when I filed that one.

Since most of the discussion is happening there, I'll close this issue as superceded by #6041.