`rustpkg build` without a package-ID argument should work

[catamorphism]

Sub-bug of #5677

On its own, rustpkg build should work if the current directory contains a main.rs, lib.rs, bench.rs, test.rs, or pkg.rs file.

[emberian]

At the very least it should print out a message, "Building from the current directory is a planned feature but not yet implemented". Current behavior is very confusing, I think it should be fixed for 0.7 (I'll do it if you don't have time / have bigger fish to fry)

[catamorphism]

@cmr Feel free to add the message. I'm hoping to fix this for 0.7 but it wouldn't hurt to have the message there just in case I don't.

[emberian]

I'll wait

[catamorphism]

Fixed pending #7419

[brson]

Not 0.7

[catamorphism]

Forgot to close this.