proc_macro: Add API for loading external files into the `SourceMap`

[camelid]

Summary

With this feature, proc macros (like askama::Template) could parse external files and then emit diagnostics with appropriate spans.

Proposed API

Other APIs are possible, but I was thinking of something like this:

impl SourceFile {
    pub fn span(&self) -> Span { ... }
    pub fn contents(&self) -> &str { ... }
    pub fn load(path: &Path) -> Self { ... }
}

I'm not super familiar with the proc_macro API, so feel free to suggest better API designs.

See also

See this discussion on Zulip and this discussion on GitHub for why I think this feature would be useful.

[camelid]

cc @djc

[camelid]

Another possible API for this is a function that could be called by a proc macro and would load a file and return a piece of AST representing that file as a string literal. In other words, this would be like include_str! that would be expanded immediately. I feel like I've seen this discussed before, but I can't remember where...

[petrochenkov]

cc #73921 #55904 rust-lang/rfcs#3200

[camelid]

Ah, that's the RFC I was thinking of!

[AlbertMarashi]

+1 on this

[Dylan-DPC]

Closing this as it is tracked in #99515