[FEATURE] Support for locally referenced packages

Pip allows for packages to be referenced locally using relative and absolute file paths (as documented in [Pip's Examples](https://pip.pypa.io/en/stable/cli/pip_install/#examples)).

This feature will ensure that all valid variants of locally referenced packages can be parsed from `requirements.txt` files.

This Feature should also resolve the following reported issues:
- #54
- #68
- #18 ?

Start by reviewing how requirements.txt entries are currently parsed and compare the supported forms with Pip's documented local package examples. Verify coverage for valid relative and absolute paths, then check the behavior against the reported cases in issues #54, #68, and #18.

Written by the indexing model from the issue text.