debug/pe: parse the import directory correctly

This parses the import table properly which allows for debug/pe
to extract import symbols from pecoffs linked with an import
table in a section named something other than ".idata"

The section names in a pecoff object aren't guaranteed to actually
mean anything, so hardcoding a search for the ".idata" section
is not guaranteed to find the import table in all shared libraries.
This resulted in debug/pe being unable to read import symbols
from some libraries.

The proper way to locate the import table is to validate the
number of data directory entries, locate the import entry, and
then use the va to identify the section containing the import
table. This patch does exactly this.

Fixes #16103.

Change-Id: I3ab6de7f896a0c56bb86c3863e504e8dd4c8faf3
GitHub-Last-Rev: ce8077cb154f18ada7a86e152ab03de813937816
GitHub-Pull-Request: golang/go#25193
Reviewed-on: https://go-review.googlesource.com/110555


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>