validate target triples before resolving crates

[brson]

I stared at this for a while

$ rustc --target=x86_64_unknown-linux-musl src/main.rs
error: can't find crate for `std` [E0463]
error: aborting due to previous error

before realizing I had typod the target triple. If the target had been validated earlier the error would be more obvious.

[japaric]

So, I checked this and

If the target had been validated earlier the error would be more obvious.

we are already doing this. See:

$ rustc --target foo bar.rs
error: Error loading target specification: Could not find specification for target "foo"

i.e. validate the target before looking for crates.

The issue here is that x86_64_unknown-linux-musl (with underscore) is accepted as a valid target (as x86_64-unknown-linux-musl) because of this function -- it s/-/_/g the target name so it can compare it to the module name, which can't have dashes in it.

The fix seems simple because that function is defined in a macro and it also has access to the target names (the ones with dashes). So we have to tweak it to use that instead of the module names (the ones without dashes). I'll look into it.