Avoid using `packages.Load` if possible

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See golang/mock#420 for an example of how one can avoid using packages.Load in mock generation. This increases performance.

Full disclosure: this would also be useful for creating a Bazel wrapper around Counterfeiter because:

• Bazel projects typically have a lot of packages in the transitive closure of dependencies, so packages.Load is a
  particularly large pain point that must be re-invoked across every package
• It might allow a partially generated GOPATH to be used instead of a complete GOPATH, which would make things even
  faster since the Bazel go_path rule can be invoked non-recursively on a small subset of the transitive closure.