Improved the use of FindInstallableFiles()

- Now a static method, for easier testing and less coupling.
- Has many method signatures. Can find files from module or stanza,
  zipfile or filename.
- Improved GetModuleContentsList() to use new FindInstallableFiles()

TODO: Still refactor everything use to use FindInstallableFiles()
(eg: Install()).

For #90, relates to #166 (which is contained by #171).