This code was fixed a while ago to ensure that xtest and fake packages came
first on the link line, but golang.org/cl/16775 added --whole-archive ...
--no-whole-archive around all the .a files and rendered this fix useless.

So, take a different approach and only put one .a file on the linker command
line for each ImportPath we see while traversing the action graph, not for each
*Package we see. The way we walk the graph ensures that we'll see the .a files
that need to be first first.

Change-Id: I137f00f129ccc9fc99f40eee885cc04cc358a62e
Reviewed-on: https://go-review.googlesource.com/21692Reviewed-by: Ian Lance Taylor <iant@golang.org>