(after discussion with Russ, trying to summarize here)
It's nice to be able to tell people to set $GOPATH == $HOME, which means their source
goes into $HOME/src and their binaries conveniently go into $HOME/bin.
The only wart with $GOPATH == $HOME is the $HOME/pkg directory, which is pretty ugly,
not useful for end users, and somewhat offensive to be littering in people's $HOME.
If issue #4443 and issue #3895 get fixed, $GOPATH/pkg might get even uglier.
Unlike $GOROOT/pkg, $GOPATH/pkg doesn't contain tool binaries. It's pretty much just a
cache.
Can't cmd/go's *.a cache go into /tmp/go-cache-$USER 0700 or ~/Library/Caches with
appropriate occasional lazy cleaning? (or letting the system clean).
