Plugin URLs should not be case-sensitive

[fin-w]

URLs should be deduplicated and matched after normalising e.g. lowercasing them, because currently it's possible to use a URL in a config and it not be registered as a plugin because the URL on dotfyle is a different case, and also it's possible to upload the same plugin more than once to dotfyle because the plugin was uploaded with two different casings in the URL.

Case in point, two plugin pages pointing to the same plugin:
https://dotfyle.com/plugins/saghen/blink.cmp
https://dotfyle.com/plugins/Saghen/blink.cmp