Apex_available check failure reveals dependency

When the check for apex_available has failed, the build system now shows
the module that brought the unavailable module into the APEX.

Bug: 151051671
Test: m
Change-Id: Id1a3fda67fe56fdc2dc90ec800d10689415de4d6

apex/apex.go

@@ -1982,7 +1982,7 @@ func (a *apexBundle) checkApexAvailability(ctx android.ModuleContext) {
 		if externalDep || to.AvailableFor(apexName) || whitelistedApexAvailable(apexName, to) {
 			return
 		}
-		ctx.ModuleErrorf("requires %q that is not available for the APEX.", to.Name())
+		ctx.ModuleErrorf("%q requires %q that is not available for the APEX.", from.Name(), to.Name())
 	})
 }