Summary

As a user I want external detection to work in parallel, so that detecting external packages is as fast as detecting compilers.

Description

Compiler detection and external package detection share the same high-level logic for the search:

1. Collect a list of candidate executables to be "checked"
2. Perform "tests" running those executables (e.g. run `gcc --version")
3. Catalogue executables on the base of the output obtained at point 2

Nonetheless the implementations are completely separate, and while compiler detection runs in parallel, external package detection does not. To achieve the goal in #39627 we should ensure feature parity between external detection and compiler detection.