Log Message:
goreturns: turn package into module-based build

I created go.mod and go.sum by running "go mod init" and "go \ 
get" in the
top-level directory and copied them over to $FILESDIR.

Also stop installing the source code and .a files. We can now also cut
the dependency on go-tools, so that go-tools can be a module. This is a
good thing, since the dependencies for the non-module build are getting
out of hand.