- common stanzas can be include other common stanzas
- `import: name1, name2` to import multiple stanzas
- Parse common stanzas in the same pass with other sections.
- Common stanzas have to be defined before use.
- Also negative tests
- Terse documentation, let's improve it as questions are asked

- Edit gen-extra-source-files to include golden files
- Amend elif warning to mention cabal-version: 2.2
- In regression golden tests, include also warnings

Note: ATM the common stanzas are completely handled inside parser,
GenericPackageDescription doesn't know about them anymore.
That can be changed, but the the flattening of
GenericPackageDescription to PackageDescription may fail.
I don't want to do that refactor now.