feat: mutable model for package data

[lars-reimann]

Closes partially #338.

Summary of Changes

• Possible replacement for current immutable model for package data. This should again simplify the development of further transformations.
• Nodes contain links to parent and to children
  • Allows to compute qualified names
  • When nodes are moved references are updated automatically
• Implementation of various tree traversals

Tasks for later

• Currently not used by anything, this will come in a future PR.
• No support for cross-references (e.g. calledAfter, which should link to Python functions)

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CREDENTIALS	secretlint	yes		no	2.25s
✅ GIT	git_diff	yes		no	0.01s
✅ KOTLIN	ktlint	7	0	0	14.99s

See errors details in artifact MegaLinter reports on GitHub Action page
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff