feat(tvix/glue/fetchers): support file:// URLs

Nix supports file:// - URLs for `fetchurl` and `fetchTarball`.

Convert the enums and function arguments to hold a URL type.
reqwest::Url is a re-export of the url crate, but they don't re-export
the parsing errors, and as we want to hold these in our Error types, add
it to Cargo.toml explicitly.

The Fetcher::download function now checks on the scheme, and either
opens the file locally, or does do a HTTP request as before.

Fetch gets its custom debug impl, removing potentially sensitive
username and password out of URLs.

Change-Id: I777db1fe487370e822cbfec4624034aca5e08045
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11504
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -4415,6 +4415,7 @@ dependencies = [
  "tvix-castore",
  "tvix-eval",
  "tvix-store",
+ "url",
  "walkdir",
  "wu-manber",
 ]