rls has racer baked in which needs to know where the rust source
is to be able to do completion for std libs. By default rls will use:
$(rustc --print sysroot)/lib/rustlib/src/rust/src
which is nonexistent, this commit sets the correct source path
in a same way like it's done in racer expression.
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
