Improve epubcheck detection

Epubcheck can be installed via package manager or otherwise made
available on a system and need not be expected in the archive-extracted
path only.
Merge branch 'bugfix-epubcheck-detection' of https://github.com/TrevorBramble/softcover into TrevorBramble-bugfix-epubcheck-detection
Refactored dependency_filename
Merge branch 'TrevorBramble-bugfix-epubcheck-detection'