go/internal/gcimporter: support vendoring

[alandonovan]

The Go 1.5. vendoring feature means that a package importer is now logically a function of two variables: the import path, and additionally the directory in which the import declaration appears.

The go/types TestStdlib test currently fails because the default importer does not support vendoring (and cannot without an API change).

[griesemer]

Duplicate of #13688.

[rsc]

@alandonovan, if I invert the flag, this gets fixed automatically, right?

[alandonovan]

No. The problem here is that vendoring makes the interpretation of an import path a function of two parameters, and the go/types.Importer interface exposes only one. Ideally, we would add a srcDir string parameter to the interface's method, and its implementations would plumb this value through to (*build.Context).Import. That, of course, would be an incompatible API change, though I suspect there are few if any clients that define their own Importer types.

(It's possible that flipping the flag causes the tests to pass, but it wouldn't solve the general problem.)