cmd/doc: be context-sensitive when resolving package paths

[robpike]

If I

$ cd $GOPATH/upspin.io
$ go doc ./errors

what comes out is the package docs for the Go standard error package, which is not the right answer. Instead, the ./ should mean to look in upspin.io/errors first.

It's even possible that if I say

$ cd $GOPATH/upspin.io/path
$ go doc errors

(no ./) it should prefer the local errors package from the upspin.io repo. Not sure about that.

But certainly ./ should mean something.

[mvdan]

@robpike should we milestone this for 1.11 if you're planning on working on it?

[gopherbot]

Change https://golang.org/cl/98396 mentions this issue: cmd/doc: make local dot-slash path names work