(MINOR) New module for XMP namespace constants

[scouten-adobe]

Changes in this pull request

Removed xmp_const module in favor of xmp_ns, which contains constants for many common XMP namespaces.

Replace xmp_const::XMP_NS_XMP with xmp_ns::XMP.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

Merging #53 (483bd7a) into main (8bf43d1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   92.13%   92.13%           
=======================================
  Files           4        4           
  Lines         267      267           
=======================================
  Hits          246      246           
  Misses         21       21           

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/xmp_file.rs	88.80% <100.00%> (ø)

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