Support Python projects

Summary:
A new root and variant `python` config field is provided with two new
fields below, `version` and `requirements`.

The former, `version`, should specify the Python executable to use when
executing related package installation commands and ostensibly the same
executable that will be used to run the application.

The latter, `requirements`, should specify all pip requirements files
such that a compiler that supports layered filesystems (e.g. Docker) can
output separate instructions that will invalidate cache layers for
changes to those files independently of changes to the rest of the
codebase.

Python related instructions will be generated only if either `version`
or `requirements` are given.

Fixes T186545

Test Plan: Run `go test ./...`.

Reviewers: thcipriani, hashar, demon, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T186545

Differential Revision: https://phabricator.wikimedia.org/D976

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