Fix makeUniquePackageNames

makeUniquePackageNames never assigned to pkgs, so all packages
were using their short names, even if another package had the
same short name.

Change-Id: I5420f7839efd25362d6c5d85265399ce7b29ddb2
diff --git a/context.go b/context.go
index 73b5e35..c6be581 100644
--- a/context.go
+++ b/context.go
@@ -1795,6 +1795,7 @@
 			// No collision so far.  Tentatively set the package's name to be
 			// its short name.
 			pkgNames[pctx] = pctx.shortName
+			pkgs[pctx.shortName] = pctx
 		}
 	}