Support allowing rewrite_imports to rewrite to URLs

[NfNitLoop]

I believe @marekbuild told me to create this issue in slack.

While buf supports a rewrite_imports option, it currently uses the : character to separate the search/replace terms of the rewrite. This means you can't rewrite to a URL, which is what I would like to be able to do in Deno. example code.

Right now, my workaround is to rewrite to a relative import, (which has its own bug), and then import and re-export the library from there.

Possible solutions:

• Only split on the first :, thus allowing there to be : characters in the replace.
• Allow escaping : characters. (But this might get a bit ugly because you might end up having to double-escape in yaml?)

[smaye81]

The rewrite_imports option was actually written with only npm in mind. We don't officially support Deno yet, so that's why this option doesn't support rewriting to a URL.

However, we know this is something that may be required in the future for full Deno support, so we will keep this open. When we address Deno at a later date, we will make sure this is part of the implementation.

[smaye81]

I am going to merge this one into #293, since that one is that main Deno support PR.