cmd/go: suppress calls to collectDeps for test packages

cmd/go: suppress calls to collectDeps for test packages

Instead, do the cycle checking in recompileForTest once the test
variant packages have been poked in the right places in the dependency
tree(graph?).

(Pair programming with bcmills@.)

For #59157.

Change-Id: I0c644cb9f2c0dac3a5b0189e2aa0eef083c669f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/482237
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>