- more orthogonal functionality of filter functions for better re-use

go/doc/doc.go:
- simplified interface
- collect filenames of packages so that they can be shown

godoc:
- removed TODO, show list of package (linked) files used to create documentation

R=rsc
DELTA=130  (68 added, 24 deleted, 38 changed)
OCL=32549
CL=32552