FetchContent: Invoke steps directly and avoid a separate sub-build

The cost of setting up and executing a separate sub-build to do the
download, update and patch steps required for FetchContent population
can be significant with some platforms and CMake generators. Avoid the
sub-build altogether by invoking the step scripts directly.

Previously, if no generator was set (e.g. population was being done in
script mode), a generator needed to be available on the default PATH.
Since we no longer use a sub-build, this restriction is also now gone.

Fixes: #21703