Refactors and warning for unlinked manuals

[saecki]

Closes #50

Initially I just wanted to implement the warning for unlinked manuals/docs, which needs both information from the manifest and the readme. Looking at the code, I thought I'd just refactor the manifest and readme checks to generate output. That got a bit out of hand, and turned into quite large changes 😅

The fundamental refactors are:

• A PackagePath struct that allows both retrieving the full path and the path relative to the package directory. This cleared things up quite a bit for me, because previously it was a little bit of a guessing game if a Path was relative or not.
• A Spanned struct that wraps a value and associates it with a Range<usize>, which is extensively used when parsing the TOML manifest.
• The manifest and readme checks return the parsed and validated information, which can be used in other checks.

I still think these refactors are worth while, and allow to more easily add more complex checks in the future.

[saecki]

I've rebased onto the integration-test branch, this PR should be merged #58.
The aero-navigator package already had an unlinked manual, so I didn't add any new reference tests.

[elegaanz]

Thank you so much for that PR, the code definitely looks much better now and will be much more pleasant to work with.