Themes: Normalize the installed theme path in...

Themes: Normalize the installed theme path in `Theme_Installer_Skin::do_overwrite()` before comparing with the uploaded theme.

This ensures that the data for the currently installed theme is picked up properly when uploading a theme update on Windows.

Follow-up to [48390].

Props bobbingwide, wpamitkumar, mukesh27.
Merges [48913] to the 5.5 branch.
Fixes #51182.
Built from https://develop.svn.wordpress.org/branches/5.5@48919


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48681 1a063a9b-81f0-0310-95a4-ce76da25c4cd