RFR: JDK-8157507: JDK-8157348 broke gensrc of imported modules

[Erik Joelsson]

In JDK-8157348 I made build time modifications of module-info.java more 
restrictive by only adding exports if the exported package actually 
exists. This existence check missed looking in imported prebuilt classes 
and because of that, importing javafx is now broken.

Bug: https://bugs.openjdk.java.net/browse/JDK-8157507
Patch:
diff -r fa3dec0c2862 make/GensrcModuleInfo.gmk
--- a/make/GensrcModuleInfo.gmk
+++ b/make/GensrcModuleInfo.gmk
@@ -93,7 +93,9 @@
    # Param 1 - Name of package with dots
    PackageExists = \
        $(strip $(wildcard $(addsuffix $(subst .,/,/$(strip $1)), \
-          $(MODULE_CLASSES_DIRS))))
+          $(MODULE_CLASSES_DIRS) \
+          $(addsuffix /$(MODULE), $(IMPORT_MODULES_CLASSES)) \
+      )))

    # Convert the modification lines into arguments for the modification 
tool.
    # Filter out modifications for non existing to-modules.

/Erik