Relax regex re. recognizing requirements.txt

[jherland]

Zhihan found that the plotly project has some requirements files that
are not recognized by FawltyDeps, with names like requirements_abc.txt.

Using the underscore as a separator in the filename does not match our
regex, which requires a word boundary ("\b") around the "requirements"
string in the filename.

Fix our regex: There is no reason why the underscore separator should
disqualify this as a requirements file, and simplifying the regex to
".*requirements.*\.(txt|in)" is unlikely to introduce false positives
(files that should not be recognized as requirements.txt files, but
now are).

Also add unit tests precisely for parsing filenames of dependency files
as I could not find another test case that tested exactly this.

[Nour-Mws]

Looks good! Hopefully we won't have any false positives popping up in the future :)