This PR fixes several issues I noticed while trying to get Spack working on Apple M1.
- [x] `build_environment.py` attempts to add `spec['foo'].libs` and `spec['foo'].headers` to our compiler wrappers for all dependencies using a try-except that ignores `NoLibrariesError` and `NoHeadersError` respectively. However, The `libs` and `headers` attributes of the Python package were erroneously using `RuntimeError` instead.
- [x] `spack external find python` (used during bootstrapping) currently has no way to determine whether or not an installation is `+shared`, so previously we would only search for static Python libs. However, most distributions including XCode/Conda/Intel ship shared Python libs. I updated `libs` to search for both shared and static (order based on variant) as a fallback.
- [x] The `headers` attribute was recursively searching in `prefix.include` for `pyconfig.h`, but this could lead to non-deterministic behavior if multiple versions of Python are installed and `pyconfig.h` files exist in multiple `<prefix>/include/pythonX.Y` locations. It's safer to search in `sysconfig.get_path('include')` instead.
- [x] The Python installation that comes with XCode is broken, and `sysconfig.get_paths` is hard-coded to return specific directories. This meant that our logic for `platlib`/`purelib`/`include` where we replace `platbase`/`base`/`installed_base` with `prefix` wasn't working and the `mkdirp` in `setup_dependent_package` was trying to create a directory in root, giving permissions issues. Even if you commented out those `mkdirp` calls, Spack would add the wrong directories to `PYTHONPATH`. Added a fallback hard-coded to `lib/pythonX.Y/site-packages` if sysconfig is broken (this is what distutils always did).
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