Fix local anchor link issue

[saswatamcode]

Currently, mdox throws an error for markdown anchor links which don't begin with # and contain a /, even though GitHub or frameworks like Hugo parse them correctly (same file or different one) such as this or this. The below errors are thrown even when file and anchor id exist,

mdox fmt -l storage.md
(6 occurrences): link design.md/#chunk, normalized to: failed to stat /Users/saswatamukherjee/web/thanos/docs/design.md/: stat /Users/saswatamukherjee/web/thanos/docs/design.md/: not a directory

mdox fmt -l design.md
(2 occurrences): link design.md/#chunk-file, normalized to: failed to stat /Users/saswatamukherjee/web/thanos/docs/design.md/: stat /Users/saswatamukherjee/web/thanos/docs/design.md/: not a directory

With this fix, the links are validated correctly and, when given options, are transformed from design.md/#chunk to design.md#chunk(if anchor is in diff file) or simply #chunk(if in same file). Test case has been added to check the same.

[bwplotka]

💪🏽 Thanks, makes sense!