Configure dependabot to watch dependencies brought in via git submodules

[scouten-adobe]

Those are libexpat and the C++ XMP Toolkit.

[codecov]

Codecov Report

Merging #48 (2a70214) into main (2a70214) will not change coverage.
The diff coverage is n/a.

❗ Current head 2a70214 differs from pull request most recent head 63668ea. Consider uploading reports for the commit 63668ea to get more accurate results

@@           Coverage Diff           @@
##             main      #48   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files           4        4           
  Lines         272      272           
=======================================
  Hits          251      251           
  Misses         21       21           

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