Fix export cache spec and fix read cached spec

[rly]

Motivation

Fix #145. Cached extensions that are not one of {core, hdmf-common, hdmf-experimental} were not being exported into the Zarr store. In addition, cached extensions were not being read correctly because the name of the ZarrSpecReader source was being set to "./namespace" rather than something unique to the spec. The specs were also not being cached. This PR makes the code for ZarrSpecReader more aligned with H5SpecReader.

How to test the behavior?

Run example code in #145 but you need to use this branch in HDMF to read the file hdmf-dev/hdmf#1205

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.58%. Comparing base (049402f) to head (bbc4ecd).

Files with missing lines	Patch %	Lines
src/hdmf_zarr/backend.py	66.66%	0 Missing and 1 partial ⚠️
src/hdmf_zarr/utils.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #232      +/-   ##
==========================================
- Coverage   85.67%   85.58%   -0.10%     
==========================================
  Files           5        5              
  Lines        1208     1214       +6     
  Branches      301      304       +3     
==========================================
+ Hits         1035     1039       +4     
  Misses        113      113              
- Partials       60       62       +2     

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[oruebel]

Looks good to me