Do not connect HTree to I/O pad if the net is internal.

* Bug: In Cumulus/plugins.chip.powerplane.Builder._connectHTree(), is was
    previously assumed that a net manged by a H-Tree was always coming from
    the outside (i.e. a clock or a reset signal).
      It is no longer the case, for example with the PLL internally generated
    clock.
      So prune internal signals in this method.

cumulus/src/plugins/alpha/chip/powerplane.py

@@ -478,6 +478,7 @@ class Builder ( object ):
                 message += [ '\n        - {} {}'.format(plug,plug.getInstance()) ]
             raise ErrorMessage( 1, message )
             return
+        if not coronaNet.isExternal(): return
         coronaPin = None
         for pin in coronaNet.getPins():
             coronaPin = pin