cmake: rework of target toolchain handling

Follow-up: #41301

This commit is a rework and cleanup of the target toolchain handling in
Zephyr CMake build system.

Instead of directly loading code a CMake modules for target toolchain
lookup, the target toolchain now follows the CMake `find_package()`
pattern for finding the target toolchain.

This makes it more clear which modules are responsible for finding tools
and which modules provides build integration / features.

The following tools can now be found using `find_package()`:
- Target toolchain: find_package(TargetTools)

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>