Several pip fixes for Python hydration

[lpsinger]

• Permit installing Python packages from source if host and target match.

  Some Python packages must be installed from source: either PyPI
  only holds source distributions of them, or lacks wheels for the
  target platform. If the host platform matches the target Lambda
  platform, then permit installation of Python packages from source.

• Use pip --version, not pip list, to test for working pip

  pip list can be very time consuming if there are many packages
  installed. pip --version is fast regardless.

• Invoke pip using python3 -m pip rather than pip3

  This is closer to the recommended way to invoke pip according to the
  latest pip documentation.

  The reason why this invocation is more robust is that it only
  requires the Python interpreter itself to be on the PATH, and it is
  robust to the bin directory corresponding to site-packages being
  omitted from the PATH (usually by accident).

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Thanks again!

[lpsinger]

Any news on this?

[ryanblock]

@lpsinger thanks for your patience! As you may have seen, my focus has been on shipping aws-lite and Arc 11; soon I'll be returning to PRs and work unrelated to that end!

[lpsinger]

Any updates on this?

[ryanblock]

Some Python packages must be installed from source: either PyPI only holds source distributions of them, or lacks wheels for the target platform. If the host platform matches the target Lambda platform, then permit installation of Python packages from source.

Yeah, I found that to be the case for some packages, but not many; my recollection at the time of building this Python integration was that the current best practice for packaging was a binary dist. Whereas the approach I took here is more rigid in its requirements for the published package, it is also significantly more deterministic, which is a quality we look for in Architect deployments. Ultimately I'm amenable to this change, but it'll need to be accompanied by at least the same level of testing rigor as we have for the other Python hydration stuff.

pip list can be very time consuming if there are many packages installed. pip --version is fast regardless.

As of this time we must run pip list to see if pipdeptree is present; this cannot be avoided. As you can see in the implementation, we only run this operation one time, then store the result in global state to prevent re-running it. Related: src/actions/autoinstall/python/check-py-tools.js is vestigial from an earlier implementation and can now be deleted; src/actions/autoinstall/python/get-dep-tree.js.

python3 -m pip... is closer to the recommended way to invoke pip according to the latest pip documentation.

Seems reasonable!

[lpsinger]

We recently split off the part of our project that was written in Python into a separate project that does not use Architect. I'm not supporting that part of our project, so I will not be taking this PR any further at the moment.