cmd/dist: ignore file names beginning with . or _
cmd/dist: ignore file names beginning with . or _

This is the same heuristic that build.ScanDir uses.
It avoids considering 'resource fork' files on OS X;
the resource for x.go is ._x.go.

R=gri
CC=golang-dev
http://codereview.appspot.com/5616073

pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang