clear out pkg tree in clean.bash.

rename place-holder to _place_holder_ so it can be avoided.

R=r
DELTA=5  (2 added, 2 deleted, 1 changed)
OCL=29093
CL=29097

src/clean.bash

@@ -3,7 +3,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-rm -f $GOROOT/pkg/*.[6a]
+rm -rf $GOROOT/pkg/[0-9a-zA-Z_]*
 rm -f $GOROOT/lib/*.[6a]
 for i in lib9 libbio libmach_amd64 libregexp cmd runtime lib
 do