linkcheck: add a 'locality' reporting field to indicate local/remote handling of hyperlinks

[jayaddison]

Is your feature request related to a problem? Please describe. The linkcheck builder is capable of checking hyperlinks that refer to network resources, and also a limited set of host-local filesystem resources.

Currently, the locality of the hyperlink is not included in the linkcheck report output; despite that, in many cases it's probably fairly clear whether each hyperlink is to a local path or remote, by inspection.

Describe the solution you'd like Although inspecting URIs is possible, I think it might be helpful to add a string-typed locality report field that indicates whether the linkcheck builder considered each hyperlink to be local or remote at runtime.

Describe alternatives you've considered

• Doing nothing; allowing users to continue to infer locality from each URI (or in some cases, implying it from status codes that only apply to non-local links).
• Using a boolean-typed field, perhaps named is_local or simply local. This would work equally well provided that locality preserves this clear distinction -- and it may well do -- but to me it feels worth having some potential for extensibility.

Additional context

• Introduction of local link handling: https://github.com/sphinx-doc/sphinx/blob/9f4c61b15415bcb219c3e238092421738b2a677e/sphinx/builders/linkcheck.py#L104-L106 (essentially: non-HTTP(S) links)
• Adaptation to presence-test for local files: https://github.com/sphinx-doc/sphinx/blob/f95ba21f4a4dd1cade2f554b1d3afa7b322be1e4/sphinx/builders/linkcheck.py#L218-L229 (not HTTP(s), and also doesn't appear to be a scheme and then a // prefix (NB: I'm not sure a // is necessarily always required for network-based resources, but it often is in practice).

[jayaddison]

Using a boolean-typed field, perhaps named is_local or simply local. This would work equally well provided that locality preserves this clear distinction -- and it may well do -- but to me it feels worth having some potential for extensibility.

Two additional locality values I can think of are: cache (according to a recent local cached result, the status of the hyperlink is ...) and archive (according to a nearby, but not necessarily same-host, archived result, the status of the hyperlink is ...).