build: don't pass --target if not needed

[davidhewitt]

Closes #229

Added a check whether the inferred target is the rust host, and if so, skip passing --target.

I did a little bit of refactoring at the same time;

• merged private and utils into a _utils module, shuffled a couple of functions around
• moved functions which query rustc for metadata into a new setuptools_rust.rustc_info module and added @cache decorator to all of these (as we expect rustc to give stable output)