* Extract the MetaPathFinder and Loaders for packages in their own classes
https://peps.python.org/pep-0451/
Currently, RepoPath and Repo implement the (deprecated) interface of
MetaPathFinder (find_module) and of Loader (load_module). This commit
extracts both of them and places the code in their own classes.
The MetaPathFinder interface is updated to contain both the deprecated
"find_module" (for Python 2.7 support) and the recommended "find_spec".
Update of the Loader interface is deferred at a subsequent commit.
* Move the lines to be prepended inside "RepoLoader"
Also adjust the naming of a few variables too
* Remove spack.util.imp, since code is only used in spack.repo
* Remove support from loading Python modules Python > 3 but < 3.5
* Remove `Repo._create_namespace`
This function was interacting badly with the MetaPathFinder
and causing issues with "normal" imports. Removing the
function allows to do things like:
```python
import spack.pkg.builtin.mpich
cls = spack.pkg.builtin.mpich.Mpich
```
* Remove code needed to trigger the Singleton evaluation
The finder is coded in a way to trigger the Singleton,
so we don't need external code now that we register it
at module level into `sys.meta_path`.
* Add unit tests
ff04d1bfc1