go.tools/gcimporter: use imported package info for anonymous fields
go.tools/gcimporter: use imported package info for anonymous fields

The package of a struct field is independent of the type of the
struct field - the old code was technically not correct. That said,
it does not seem possible (or very difficult) to create a test case
because for exported anonymous fields that field package doesn't matter
(it's not needed for name identity), and non-exported anonymous fields
cannot be accessed from an imported package.

R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/47560043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old