cmd/go: drop $GOPATH/pkg

[bradfitz]

(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).

[adg]

Comment 1:

Do you propose to change the behavior for everyone, or make this configurable?
What about people that want to ship .a binaries only? I think the go tool currently
supports that. (ie, just mkdir the src/ path and put the binaries under pkg/ and it
works)

[minux]

Comment 2:

FYI, for binary-only package to work, you will need to create a dummy "package xxx" Go
source
file in the directory with an older timestamp than the .a file.
This is issue #2775.

[rsc]

Comment 3:

There are many facets to think about. This is just an issue to remind us to
do so.

[rsc]

Comment 4:

Labels changed: added priority-later, go1.1maybe, removed priority-triage, go1.1.

Status changed to Thinking.

[robpike]

Comment 5:

Labels changed: removed go1.1maybe.

[rsc]

Comment 6:

Labels changed: added go1.3.

[robpike]

Comment 7:

Labels changed: removed go1.3.

[rsc]

Comment 8:

Labels changed: added go1.3maybe.

[rsc]

Comment 9:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 10:

Labels changed: added repo-main.