Enhance python cataloger to be image/directory aware.

[wagoodman]

In order to reduce the number of false positives when scanning images while also keeping the value of grype-vscode [directory] scans, it would be useful when scanning directories to key off of index-like files (e.g. requirements.txt) and installation metadata (e.g. egg, wheel, setup.py) when scanning images for the python cataloger.

AC

• Dependencies described within requirements.txt-like files are not shown during image scans (but are shown during directory scans)
• All other python catalog sources are reported for both image and directory scans.

Steps to Test

• When scanning a directory:
  • ensure there are examples of a requirements.txt, setup.py, egg metadata installations, and wheel metadata installations in the directory being scanned.
  • ensure the scan finds all python examples.
• When scanning an image:
  • ensure there are examples of a requirements.txt, setup.py, egg metadata installations, and wheel metadata installations in the image being scanned.
  • ensure the scan finds all python examples EXCEPT any requirements.txt references.

Notes for Developer
Should mirror the gemspec cataloger additions made in #203