cmd/go,testdeps: move import of internal/coverage/cfile to testmain

cmd/go,testdeps: move import of internal/coverage/cfile to testmain

Instead of having testing/internal/testdeps import the
internal/coverage/cfile package directly, have the code in testmain
pass in pointers to cfile functions during setup in the case that
we're running a "go test -cover" binary. This reduces the size of
regular non-coverage test binaries back to what they were before CL
585820.

Updates #67401.
Fixes #67588.

Change-Id: Iaf1a613bc7d3c9df9943189065d0161ca9120d34
Reviewed-on: https://go-review.googlesource.com/c/go/+/587795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>