Improve Spack's parsing of module show to eliminate some false
positives (e.g. accepting MODULEPATH when it is in fact looking for
PATH). This makes the following changes:
* Updates the pattern searching for several paths to avoid the case
where they are prefixes of unwanted paths
* Adds a warning message when an extracted path doesn't exist (which
may help catch future module parsing bugs faster)
* Adds a test with the content mentioned in #9083
22fbb3dba7