- Changed filter function for parser.ParsePackage to
  take an *os.Dir instead of a filename for more
  powerful filters

- Removed TODO in ast.PackageInterface: Now collect
  package comments from all package files

- Cleanups in godoc: Use the new ParsePackage and
  PackageInterface functions; as a result computing
  package information is much simpler now.

R=rsc
DELTA=285  (80 added, 110 deleted, 95 changed)
OCL=32473
CL=32486