cmd/api: allow use with external packages

[bamiaux]

I'd like to be able to use go tool api on external packages and not just with the
standard go libray.
Something like 'go tool api ...mypackage' which generates a text file containing
mypackage public api.
This would be useful to prevent unecessary API breakage.

[rsc]

Comment 1:

Sounds reasonable but I think it's too late for Go 1.1.

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

Status changed to Accepted.

[rsc]

Comment 2:

Labels changed: added go1.2maybe, suggested.

[robpike]

Comment 3:

Unlikely to happen in Go1.2

Labels changed: added go1.3maybe, removed go1.2maybe.

[robpike]

Comment 4:

Labels changed: removed go1.3maybe.

[rsc]

Comment 5:

Labels changed: added go1.3maybe.

[rsc]

Comment 6:

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

[rsc]

Comment 7:

Labels changed: added repo-main.

[gopherbot]

Comment 8 by vbatts:

Attached is a patch that enables this, and does not break current usage of the tool.
It adds a -gopath flag to enable searching of GOPATH.
While it could list all the api found in GOPATH with no additional arguments, the walker
finds source that have errors (typechecking, etc). So the best use case for specific
searches of a package (i.e. `api -gopath github.com/me/hotpackage`)
This patch is based on 18815:21fb28e644a1 tip, and uses
https://golang.org/issue/7103 to be able to build cmd/api

Attachments:

1. cmd-api_includeGopath.patch (3353 bytes)