Persist pip-source layer; add layer path to PIP_FIND_LINKS env var

[thitch97]

Summary

The changes in this PR ensure that we persist the extracted pip source code in a layer which is available to downstream buildpacks at build-time instead of an ephemeral directory so that invocations of pip install may reliably access the packages bundled therein (setuptools, wheel, etc.). Setting the PIP_FIND_LINKS environment variable ensures that the pip-install buildpack can see the location of the pip source without explicit coupling.

Use Cases

Resolves #421

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[robdimsdale]

Looks great - thank you!