Import hooks using Python's built-in machinery (#23288)

The function we coded in Spack to load Python modules with arbitrary
names from a file seem to have issues with local imports. For
loading hooks though it is unnecessary to use such functions, since
we don't care to bind a custom name to a module nor we have to load
it from an unknown location.

This PR thus modifies spack.hook in the following ways:

- Use __import__ instead of spack.util.imp.load_source (this
  addresses #20005)
- Sync module docstring with all the hooks we have
- Avoid using memoization in a module function
- Marked with a leading underscore all the names that are supposed
  to stay local

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