Don't ship duplicate sets of libraries

[alexcrichton]

Today when you install Rust by default you get duplicate sets of libraries at $sysroot/lib and $sysroot/lib/rustlib/$host/lib, for example:

$ find . -name libproc_macro-* | xargs shasum
2453aab91f0cd5e77b9f514d5df15be7b4ec965e  ./lib/libproc_macro-2641322a715c6559.dylib
2453aab91f0cd5e77b9f514d5df15be7b4ec965e  ./lib/rustlib/x86_64-apple-darwin/lib/libproc_macro-2641322a715c6559.dylib

We should avoid duplicating these libraries! Unfortunately I'm not sure of the best method of doing so.

[brson]

Moving to an on disk scheme that doesn't have duplicate paths to the bins seems fairly challenging, but I can easily imagine installers being smartened to remove the duplication as best they can.

[Mark-Simulacrum]

closing in favor of #42645