matchFileGlob should only monitor for existence

Since the matchFileGlob function returns names of files and not content,
actions using it don't actually depend on the content, so it's more
consistent not to monitor the content.

In particular, following Edward's recent fix, the code that reads the
package cabal files already monitors the content. This is the
appropriate separation of concerns, the code that finds the .cabal files
monitors the search glob for file existence, while the code that reads
the .cabal files monitors the file content.