Main: use .spec file if no .cabal file to determine package

helper functions
- fileWithExtension: unique file in DIR with EXT
- fileWithExtension_: whether there is unique file in DIR with EXT
- cabalFromSpec: reads package name from SPEC and calls findCabalFile
- removePrefix: removes PREFIX from STRING

now only dies if no .cabal or .spec file found
Main: also handle spec file arg; warn if > 1 .cabal or .spec file in dir