cmd/dist: read dependencies from source files

I do not remember why we require deps.go to have a hard-coded
copy of the dependency information for cmd/go, when we can
read it from the source files instead. The answer probably involves
cmd/dist once being a C program.

In any event, stop doing that, which will eliminate the builder-only
failures in the builder-only deps test.

Change-Id: I0abd384c47401940ca08427b5be544812edcbe7f
Reviewed-on: https://go-review.googlesource.com/76021


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>