Edward Z. Yang authored Dec 27, 2016 and Edward Z. Yang committed Jan 02, 2017

A signature package is a package that contains only signatures but no
modules (equivalently, it is a package that has no provisions.)

Previously, we did not treat signature packages any differently from
normal packages, which, in particular, meant that when we instantiated a
package using a signature package, we also instantiated the signature
package.  This is actually pretty useless, because we never actually use
the instantiated signature (there's no code that actually gets compiled
in this case).

This patch treats signature packages specially, so that we don't
actually instantiate dependencies on them.  This means we have
to compile less packages when handling signatures.  Good!

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>