Refactor: split into packages

[Hellgartner]

Summary of changes

Splitting up the library into sub-libraries as a preparation for adding new input file formats

Context and reason for change

• There are a coupe of plans to add new input formats into the library, e.g. ScanCode parsing and conversion #50
• This requires introducing a clear boundary where the current SPDX specific code ends and the opossum specific code starts
• This is the first part of that effort

How can the changes be tested

• should be mostly covered by ci
• Convert a more sophisticated SPDX file with current master and the new version. Results should be identical.

[abraemer]

LGTM