Replace go list

Main motivation is so that we can attempt to pick up packages across
architectures and to have more control over the list of packages.

I consider this the first in several structural changes.

Most additions are inside of list.go.

For each run of listPackage a depScanner is made to track already seen
packages (inspired by Scanner). That list is not cached between
executions.

The current implementation of listPackage attempts to resolve vendor/ a
bit better than we used to, but other changes need to be implemented
elsewhere in godep before that can be fully realized.

All *existing* tests pass, but this doesn't mean it actually works
*better*.

A test was added to show that we fix #271.

Changelog.md

@@ -1,3 +1,7 @@
+# v33 2015/12/07
+
+* Replace the use of `go list`. This is a large change although all existing tests pass.
+
 # v32 2015/12/02
 
 * Eval Symlinks in Contains() check.