Add popular utility package mappings (attrs, python-docx, XlsxWriter)

[Johnnnmai]

Adds mappings for widely-used utility packages. For example, import attr requires attrs on PyPI, and import docx requires python-docx.

[PNHD]

Two of these one-to-one mappings are ambiguous and can produce incorrect requirements. attr is itself a distinct PyPI distribution that provides from attr import attr (and explicitly notes that attrs also provides/shadows an attr package), while both whois and python-whois are separate distributions using the whois import name. Hard-mapping attr -> attrs and whois -> python-whois therefore rewrites valid dependencies to different projects. #141 already tracks this one-import-to-multiple-distributions problem. Please remove/split the ambiguous entries unless the resolver can disambiguate them; the unambiguous mappings can be reviewed separately.