TMI: Adventures in the front-end

Still working on #5681, external crate inference. I resolved one of the remaining problems, which was how to not try to download-and-build std in directives like extern mod std; (where std is obviously already installed). But now, much to my dismay, I'm seeing some strange rustc behavior:

<core-macros>:1:0: 1:0 error: duplicate definition of type `__test`
<core-macros>:1 pub mod macros {
                ^
<core-macros>:1:0: 1:0 note: first definition of type __test here:
<core-macros>:1 pub mod macros {

This is only when compiling a file with a #[test] in it. What's weird is I haven't changed anything affecting how I interface with rustc. I'm not sure where the duplicate definitions of __test (the name that gets used for the automatically-created test module) are coming from, nor do I have any clear idea how to debug this.

Otherwise, today was bug triage and cleaning up warnings and stuff, in the moments when rustpkg was recompiling (of which there were many, since the compiler seems to have gotten quite a bit slower over the long weekend).

Better luck tomorrow, maybe...