BuildTarget: get_all_link_deps() to get_runtime_dependencies()

[xclaesse]

We already have get_dependencies() which are libs needed to link the
target. get_all_link_deps() name was thus confusing, what it actually
does is returning runtime dependencies, i.e. shared libraries that are
needed to run an executable.

This fix:
- runtime dependencies could be hidden behind link_whole targets that
  where previously ignored.
- Include CustomTarget that links_dynamically(), i.e. a DLL built by
  custom target.
- get_link_deps_mapping() and get_link_dep_subdirs() now calls
  get_runtime_dependencies() to avoid recursing multiple times. We can
  reuse the cached result in a single method.
- determine_windows_extra_paths():
  * Only add to PATH targets that links dynamically, some static
    libraries were previously included.
  * Exclude executable's build directory because Windows search there by
    default already (Fixes #12042).
  * mingw extra paths are also needed if python script has a cross built
    executable in its args.
  * External DLL paths are needed in the case a python script has a
    build targets in its args.