Allow adding a default source with a base URI of None

[handrews]

Minimally fixes #75. This can be used to match complete URIs, particularly URN-ish URIs using schemes that do not readily break down into a base and a relative reference.

The details of the change (using a string prefix after verifying the base URI when it is not None) is invisible to the public API, and the fundamental semantics are preserved.

This is sufficient for the use case that was blocking me: how to load a URI like tag:modern-json-schema.com,2023-03:something, which cannot be split. Its only components are the scheme tag and the path modern-json-schema.con,2023-03:something, and that path cannot be used as the start of a relative reference because it contains a :.

Potential workarounds

An alternative would be to register a source for each expected URI using the whole URI as a base URI, but that wouldn't work for URIs not known in advance.

Discarded approaches

I attempted the more flexible uri_prefix idea proposed in #75, but it became complicated and required awkward contortions to maintain backwards compatibility in the public API. I decided that the extra flexibility was not worthwhile as a source registered in this way can do whatever re-mapping it wants with the relative_path, such as converting : characters to something else so that the URI can be used on Mac OS X filesystems.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (0c210c2) 92.72% compared to head (6812b6e) 92.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   92.72%   92.73%   +0.01%     
==========================================
  Files          23       23              
  Lines        2049     2052       +3     
  Branches      434      435       +1     
==========================================
+ Hits         1900     1903       +3     
  Misses         97       97              
  Partials       52       52              

Impacted Files	Coverage Δ
jschon/catalog/__init__.py	98.17% <100.00%> (+0.03%)	⬆️

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[marksparkza]

I'm happy with this approach. It makes sense for base_uri=None to imply a default source; this just needs to be clarified in the docstring. Comments coming up...