Use 'installer' for unpacking wheels into the venv

[mattem]

Currently we assume there's a pip installed in the same bin directory as python, and we assume the version is compatible with the flags passed. If using the hermetic toolchains from rules_python this is generally okay, however if using a different toolchain or that of the host, then we can not guarantee the presence of pip where we expect it.

An alternative would be to use https://github.com/pypa/installer directly and not have pip involved.

Another alternative would be to not to use either for actions, and have bazel layout the venv using the runfiles symlinks.

[mattem]

Closed via #233