WP_Filesystem: Use the FTP_* path override constants during upgrades for prefixed paths, ie. use FTP_BASE for ABSPATH/sub-dir as well as just for ABSPATH/. Props vericgar for initial patch. See #14401

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Correct a typo in a docblock in [25055]. Props buffler. See #16754

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Allow for plugin translations to be loaded from WP_LANG_DIR/plugins/$domain-$locale.mo.

props dimadin.
see #18200.


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