Add metadata handling for pip packages

[stefan6419846]

We already analyze the metadata of font and RPM files and could extend this to Python packages as well. Some basic implementations are available from https://github.com/stefan6419846/pip-licenses-lib

Some things which could be considered:

• Retrieve name, version, author/maintainer, homepage, licenses, license file, dependencies
• Basic verification tasks
  • At least author or maintainer is set
  • Homepage is set and an URL
  • At least one license is declared and no full license text
  • At least one license file is available