Allow customization of fetch

[salmenf]

For my use case, I am trying to set up resolving certain installs of packages through the FileSystem API. With a custom provider, generating the right URLs is straighforward, but I run into Module not found errors once I try to run install, of course (since the files are not accessible via any URL).

It would be great if I could overwrite fetch to give custom responses. This could be global (extending fetchOptions) or tied to the provider. Then, I could for example respond to fetch("my/local/url") by accessing the File System through the FileSystem API, creating a Response object and returning it.

I appreciate the previous changes, by the way, my use case is really coming together thanks to your work! :)

[guybedford]

This is a good suggestion, I've put something together in #385, let me know if that can work for you.

[salmenf]

That's great, thank you! Should work perfectly for me: I can setFetch with my own implementation where I match the given URL and perform my file system access, or fall back to the default fetch implementation for all other URLs.