mesonbuild: move subdir generation along link dep generation

The problem with the earlier position of the generation code was, that
the results could not be cached, because the list of all link_deps was
overall different. However, it shared a special kind of subsets with
other build build targets.

Generating the set of subdirs that are required for linking, alongside
with the link dependencies brings the possibility of caching this.
This reduces the buildting from 1 min. in efl down to 20 sec. And
reduces the amount of 30872534 calls down.

this saves ~40 sec.

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