Use the non-deprecated MetaPathFinder interface (#29745)

* 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

lib/spack/llnl/util/lang.py

@@ -889,11 +889,6 @@ def load_module_from_file(module_name, module_path):
             except KeyError:
                 pass
             raise
-    elif sys.version_info[0] == 3 and sys.version_info[1] < 5:
-        import importlib.machinery
-        loader = importlib.machinery.SourceFileLoader(  # novm
-            module_name, module_path)
-        module = loader.load_module()
     elif sys.version_info[0] == 2:
         import imp
         module = imp.load_source(module_name, module_path)