minimize the rust-std component

This splits out a rustc-dev component with the compiler crates, and
keeps the status quo of default installed files on nightly. The default
changing to not install compiler libraries by default is left for a
future pull request.

However, on stable and beta, this does remove the compiler libraries
from the set of libraries installed by default, as they are never needed
there (per our stability story, they "cannot" be used).

src/bootstrap/builder.rs

@@ -443,6 +443,7 @@ impl<'a> Builder<'a> {
                 dist::Rustc,
                 dist::DebuggerScripts,
                 dist::Std,
+                dist::RustcDev,
                 dist::Analysis,
                 dist::Src,
                 dist::PlainSourceTarball,