It's common to use the go list command in shell scripts, but
currently it's awkward to print a string slice from the Package
type in a way that's easily parseable by the shell.  For example:

        go list -f '{{range .Deps}}{{.}}
        {{end}}'

(and even that prints an unwanted new line at the end|).

To make this easier, this CL adds a "join" function to the
format template.

        go list -f '{{join .Deps "\n"}}'

R=rsc, dsymonds, minux.ma, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/6680044