golang/xenlight: Get rid of GOPATH-based build artefacts

The original build setup used a "fake GOPATH" in tools/golang to test
the mechanism of building from go package files installed on a
filesystem.  With the move to modules, this isn't necessary, and leads
to potentially confusing directories being created.  (I.e., it might
not be obvious that files under tools/golang/src shouldn't be edited.)

Get rid of the code that creates this (now unused) intermediate
directory.  Add direct dependencies from 'build' onto the source
files.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Nick Rosbrook <rosbrookn@ainfosec.com>

tools/Rules.mk

@@ -35,7 +35,6 @@ XENSTORE_XENSTORED ?= y
 debug ?= y
 debug_symbols ?= $(debug)
 
-XEN_GOPATH        = $(XEN_ROOT)/tools/golang
 XEN_GOCODE_URL    = golang.xenproject.org
 
 ifeq ($(debug_symbols),y)