cmd/addr2line, cmd/nm: factor object reading into cmd/internal/objfile

To do in another CL: make cmd/objdump use cmd/internal/objfile too.

There is a package placement decision in this CL:
cmd/internal/objfile instead of internal/objfile.
I chose to put internal under cmd to make clear (and enforce)
that no standard library packages should use this
(it's a bit dependency-heavy).

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/123910043
go/build: look in $GOROOT/src/cmd/foo/bar for import cmd/foo/bar

This lets us have non-main packages like cmd/internal or cmd/nm/internal/whatever.

The src/pkg migration (see golang.org/s/go14mainrepo) will allow this
as a natural side effect. The explicit change here just allows use of the
effect a little sooner.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/117630043