Include package name/version in build script warnings.

[ehuss]

Describe the problem you are trying to solve
In some situations, it is not clear where a warning is coming from when emitted from a build script. For example, a build script like this:

fn main() {
    println!("cargo:warning=this is a warning");
}

displays:

warning: this is a warning

Describe the solution you'd like
I think it might be helpful to prefix the warning with the package name and version (similar to how -vv output is). Perhaps something like this:

warning: [pkg_name 1.2.3] this is a warning

Notes
Warnings are emitted here. The package name/version should be accessible from the unit.