build: make 'go get github.com/cockroachdb/cockroach' fail

[benesch]

The root package, github.com/cockroachdb/cockroach, previously served as
the main entry point for the default CCL binary. This meant that naively
running

$ go get github.com/cockroachdb/cockroach

would fail with cryptic errors about missing "reserved keywords", as our
build requires generating some Go code via Make. Our documentation
suggests that users instead run

$ go get -d github.com/cockroachdb/cockroach
$ make

but many users, somewhat understandably, fail to read that
documentation.

Move the main entry point to ./pkg/cmd/cockroach. Running 'go get
github.com/cockroachdb/cockroach' will now produce a less confusing
error that reads:

can't load package: package github.com/cockroachdb/cockroach: no Go
files in GOPATH/src/github.com/cockroachdb/cockroach

Hopefully this spurs users to read the build instructions.

(Running 'go get github.com/cockroachdb/cockroach/pkg/cmd/cockroach'
still produces the same cryptic error messages as before, but users are
far less likely to construct that 'go get' invocation.)

Fix #23583.
Fix #23992.

Release note: None

[cockroach-teamcity]

This change is 

[bdarnell]

This may warrant a PSA on the mailing list, because I think if you've set the "scope" for go-guru in your editor to github.com/cockroachdb/cockroach you'll need to update it to point to the new root. (although I'm apparently wrong about how this works, because I don't have go-guru-scope set in my editor currently. I thought it was required)

[benesch]

@knz I've been holding off on merging this because I'll be on vacation for the next two weeks and won't have time to address the fallout. If you're willing to take responsibility, though, please feel free to notify the mailing list and merge!

[benesch]

bors r=bdarnell

[craig]

Build succeeded

• GitHub CI (Cockroach)

[rendaw]

So, to get the fallout started:

The docs still say

2. Get the CockroachDB code:
go get -d github.com/cockroachdb/cockroach
cd $(go env GOPATH)/src/github.com/cockroachdb/cockroach

but the first step fails (just broke an automated build I'm working on).

What should one do now? mkdir -p /go/src/github.com/cockroachdb && git clone https://github.com/cockroachdb/cockroach.git /go/src/github.com/cockroachdb/cockroach?

As an additional note, as go get doesn't support version pinning so any scripts that used that (even if they eventually built off a tag) have probably been broken by this. Not sure how widespread that practice is though.