Fix old I03 data not processing

[ndevenish]

This data did not process, because:

• dxtbx could not find the _meta.h5 file (it was not linked from the .nxs, which was assumed). There is now a fallback that uses pure filename instead of trying to determine it from the main file.
• The _meta.h5 file didn't have a stored "Detector bit depth" field. This now uses the per-image bitdepth field.
• General cleanup/update to pathlib.

• Fixes dials/dials#2669.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 41.73%. Comparing base (78147cc) to head (d0934b3). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #737 +/- ## ========================================== - Coverage 42.15% 41.73% -0.42% ========================================== Files 188 188 Lines 16764 16877 +113 Branches 3196 3232 +36 ========================================== - Hits 7067 7044 -23 - Misses 9052 9178 +126 - Partials 645 655 +10 ```

[dagewa]

Thanks, confirmed I can process that data set now 👍