goinstall: report every newly installed package to the dashboard

This makes "goinstall -a" work on systems with GOROOTs that are
not user-writable, as is the case with Debian's Go packages.

This also makes goinstall.log the canonical list of installed
packages, in that only packages new to goinstall.log are reported to
the dashboard.

A side-effect is that writing to goinstall.log is now mandatory.
(A bug in the original implementation meant this was the case, anyway.)

The principal benefit of this change is that multiple packages from the
same repository can now be reported to the dashboard.  It is also less
likely for a user to report multiple installations of the same package
to the dashboard (they would need to remove the package from
goinstall.log first).

R=rsc, n13m3y3r
CC=golang-dev
https://golang.org/cl/4786041