exp/template: construct sets from plain template files

exp/template: construct sets from plain template files
This is the last piece (I hope) of the set creation code.
These helpers create sets from files containing individual
template definitions, free of {{define}} clauses. This
design is helpful if the templates live one per file,
undecorated.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4801052