Swallow type errors if possible

[navijation]

Fixes

• Avoid fast-failing on unresolved imports that aren't used in the target interface #182

Summary

In the event that a source package contains type errors, refrain from fast-failing immediately upon package load.
Defer the failure to the point at which the target interface's methods are analyzed and found to contain invalid
types.

Motivation

packages.Load tends to be a performance bottleneck for larger projects (hence #181). However, it seems that removing
it require a large architectural change to counterfeiter internals, namely analysis of file ASTs to create a more limited,
lazy version of packages.Load.

Certain build systems, namely Bazel, can limit the cost of package loading by sandboxing a GOPATH much smaller than
the entire dependency tree of a package and running Counterfeiter in the sandbox. However, Counterfeiter crashes if any
import in the target package is unresolved, which limits the extent of this optimization.

Downsides

• Creating tests with non-compiling packages seems rather complex (see changes to script/ci.sh) and might need
  to be thought out more carefully
• The package loader doesn't seem to preserve unresolved symbols from the AST, so error messages list an
  erroneous signature as MethodName(param1 Type1, invalid type, invalid type)(invalid type, error). This might be
  the best option short of custom AST analysis.