godoc: make ?m=src mode deterministic

Merge package files in the go/ast MergePackageFiles
function always	    in the same order (sorted by filename)
instead	 of map iteration order to obtain the same
package	 file each time.  This functionality is used
by godoc when displaying packages in ?m=src mode.

Also: minor cleanup in godoc.go.

R=rsc
CC=golang-dev
https://golang.org/cl/5540054