Ensures that string lookup in MO files only uses the singular string.
This matches expected behavior with gettext files and improves compatibility for cases where for example both `__( 'Product' )` and `_n( 'Product', 'Products’, num )` are used in a project, where both will use the same translation for the singular version. Maintains backward compatibility and feature parity with the pomo library and the PHP translation file format.
Replaces [57386], which was reverted in [57505], with a more accurate and performant solution.
See #59656.
git-svn-id: https://develop.svn.wordpress.org/trunk@57513 602fd350-edb4-49c9-b593-d223f7449a82
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