(apro) build-aux: go-mod.mk: Simplify build rule, export go.bin.rule function
(apro) build-aux: go-mod.mk: Simplify build rule, export go.bin.rule function

 - Take advantage of $(COPY_IFCHANGED) and simplify the _go.bin.rule definition
 - Don't list .cache.BIN as a separate target.  This will be useful
   later so that we can have "sidecar" commands that inherit the
   environment properly.
 - Have _go.bin.rule take the name and package as arguments, instead
   of hard-coding expressions based on $(go.bin).
 - Rename _go.bin.rule to go.bin.rule, and semi-document it.  This
   will allow code-reuse for the Lyft binaries in the APro Makefile.

datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador