rustPlatform: add buildAndTestSubdir-argument

There are several tarballs (such as the `rust-lang/rust`-source) with a
`Cargo.toml` at root and several sub-packages (with their own Cargo.toml)
without using workspaces[1].

In such a case it's needed to move into a subdir to only build the
specified sub-package (e.g. `rustfmt` or `rsl`), however the artifacts
are at `/target` in the root-dir of the build environment. This breaks
the build since `buildRustPackage` searches for executables in `target`
(which is at the build-env's root) at the end of the `buildPhase`.

With the optional `buildAndTestSubdir`-argument, the builder moves into
the specified subdir using `pushd`/`popd` during `buildPhase` and
`checkPhase`.

Also moved the logic to find executables and libs to the end of the `buildPhase`
from a custom `postBuild`-hook to fix packages with custom `build`/`install`-procedures
such as `uutils-coreutils`.

[1] https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html

pkgs/development/compilers/rust/rls/default.nix

@@ -10,8 +10,9 @@ rustPlatform.buildRustPackage {
   dontUpdateAutotoolsGnuConfigScripts = true;
 
   cargoVendorDir = "vendor";
+  buildAndTestSubdir = "src/tools/rls";
+
   preBuild = ''
-    pushd src/tools/rls
     # client tests are flaky
     rm tests/client.rs
   '';
@@ -28,8 +29,6 @@ rustPlatform.buildRustPackage {
 
   doCheck = true;
 
-  preInstall = "popd";
-
   doInstallCheck = true;
   installCheckPhase = ''
     $out/bin/rls --version