[19.09] buildRustCrate: add support for renaming crates

[andir]

Motivation for this change

19.09 version of #68296. This should be zero rebuilds.

Before this change, buildRustCrate always called rustc with

--extern libName=[...]libName[...]

However, Cargo permits using a different name under which a dependency
is known to a crate. For example, rand 0.7.0 uses:

[dependencies]
getrandom_package = { version = "0.1.1", package = "getrandom", optional = true }

Which introduces the getrandom dependency such that it is known as
getrandom_package to the rand crate. In this case, the correct extern
flag is of the form

--extern getrandom_package=[...]getrandom[...]

which is currently not supported. In order to support such cases, this
change introduces a crateRenames argument to buildRustCrate. This
argument is an attribute set of dependencies that should be renamed. In
this case, crateRenames would be:

{
"getrandom" = "getrandom_package";
}

The extern options are then built such that if the libName occurs as
an attribute in this set, it value will be used as the local
name. Otherwise libName will be used as before.

(cherry picked from commit 85c6d72)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @danieldk @gilligan
cc release manager @disassembler @lheckemann