* Extracts logic to find execroots as a separate method and moves it into the `collectLibrariesToLink()` method from the constructor.
* Extracts logic to create rpaths for toolchain libraries as a separate method.
* Reorganizes the logic for better readability.

This debloats the constructor and makes further changes easier.

When sibling repository layout is used, the toolchain solib directory is not co-located with the main solib (solib_<cpu>). Therefore, the RPATHs for toolchain solib need to be computed separately, in a similar manner as how they are computed for the main solib.

This change uses a private implementation of `getRelative()` function, rather than the `PathFragment.relativeTo()` method. The reason is that `PathFragment.relativeTo()` requires the base path to be a prefix which is not true in the cases here.

Fixes #16956