Declare the uri-polyfill package as provide ext-uri

[stof]

This allows installing this polyfill package to satisfy a requirement on ext-uri of another package.

When a package provides a polyfill for all of an extension, this is the best way to implement such polyfill. For instance, Symfony does that for its iconv or mbstring polyfills.

[nyamsprod]

thanks I overlooked this. I will merge it ASAP