In plugins and every program that opens a plugin, include a hash of
every imported package.

There are two versions of each hash: one local and one exported.
As the program starts and plugins are loaded, the first exported
symbol for each package becomes the canonical version.

Any subsequent plugin's local package hash symbol has to match the
canonical version.

Fixes #17832

Change-Id: I4e62c8e1729d322e14b1673bada40fa7a74ea8bc
Reviewed-on: https://go-review.googlesource.com/33161Reviewed-by: Ian Lance Taylor <iant@golang.org>