Fix source of consistent resolution being eagerly resolved

[melix]

This commit fixes an issue with consistent resolution, which caused
the consistent resolution source to be fully resolved (including
external dependencies) during task dependencies computation.

To fix this problem, we reworked how "synthetic" dependencies are
added to the resolution process. Synthetic dependencies include
dependency locking constraints, as well as constraints for
dependency resolution consistency. They would now only be added
to the resolution graph if we're actually at the execution phase,
not during task dependency computation.

Fixes #15588

Fixes #?

Context

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[melix]

@bot-gradle test this

[bot-gradle]

OK, I've already triggered ReadyForMerge build for you.

[ljacomet]

LGTM!