Simplify war rule

Current implementation makes assumption for the location of context
content artifacts: one sub directory level. This is not future
proof as migration to built in gwt_binary() has shown.

Move from inducing the location to use of Buck's own $(location)
macro and provide the paramter to pack_war.py through cmd rather
than srcs.

Change-Id: I69037920e21963a62c8edd6edc296d72c7454e5a