Explicitly support implicit build deps

[jaraco]

This change uses the --use-pep517 flag in a parameter to pip to ensure that any projects that implicitly require Setuptools will get it, removing the implicit dependency on setuptools in the user's environment.

Additionally, explicitly declares the dependency on pip and invokes it from the environment where vendoring is installed rather than relying on the user to have pip installed somewhere in their system path.

Fixes the issue reported in #39.

setuptools feature/vendoring $ pip-run -q git+https://github.com/jaraco/vendoring@bugfix/39-explicit-build-deps -- -m vendoring sync
Working in .
Load configuration... Done!
Clean existing libraries... Done!
Add vendored libraries... Done!
Fetch licenses... Done!
Generate static-typing stubs... Done!

[pradyunsg]

Happy to merge this, whenever you mark this as ready for review. :)

[jaraco]

Yep. Thanks for the reminder. I marked it draft when I'd noticed I'd missed one of the pip calls, since corrected and now ready.

[pradyunsg]

Closing this since the underlying issue has been fixed in pip.