Rustpkg should extract dependencies from `extern mod` statements.

[graydon]

Sub-bug of #5677

Rustpkg currently parses pkg.rs to figure out dependencies between packages. This has to change:

• As discussed in Rustpkg should infer crates to build #5680 rustpkg should usually operate without any pkg.rs files. It needs to find dependencies by inference by default. This means reading the source files and looking for extern mod statements (post-syntax-expansion and post-cfg-filtering).
• As discussed in Rustpkg should operate on a URL-like pkgid #5679 the strings found in extern mod statements should be pkgids, which should be enough to find and install a package through a variety of mechanisms (RUST_PATH search order, falling back to local binary package management and/or fetching source from a URL)

[catamorphism]

I opened a separate bug, #6407, for the second bullet point. I'm going to implement this issue first.

[catamorphism]

Er, "this issue" as in inference; then I'll expand what's allowed in extern mod directives.

[catamorphism]

The first part of this is done, except for cleaning up tidy errors and figuring out how to work around a borrow check issue that I'm currently handling with a copy. I might just submit a pull request with the latter issue unfixed and request advice.