Discover requirements from other sources than Pip requirement file

[sveetch]

Is your feature request related to a problem? Please describe.

Currently, this tool only get its requirements to analyze from Pip requirements syntax, either from a file or stdin.

Describe the solution you'd like

There is many more ways to define project requirements than the Pip requirements file. Even the Pip requirement file is something that would work in any environment because in last resort the "pip freeze" command will be enough, it would be
simpler to discover requirements from other sources like "setup.cfg" or "pyproject.toml".

Describe alternatives you've considered

Instead of implementing discovery for various other sources, we could just add a single alternative to the Pip requirement file using the "packaging" way where could target a package name. With "packaging" library we could easily retrieve its requirements but obviously it means it would have to be installed.

Additional context

Since other sources have the concept of "extra requirements" (dev, doc, quality, etc..) we will need options to include/exclude them from discovery, on default they would all be ignored and only be processed if included.