PluginFactory is a function that returns a plugin. We have been
comparing these functions in unit tests and it has worked so far, but
starts to fail in gotip/master.
Note from the golang team:
```
Func values are incomparable. It is true that you could get the PC
through reflection but it is still not expected to be compared. PCs
can change due to inlining, wrappers, etc., depending on the
compiler's details, which is subject to change.
```
Signed-off-by: Davanum Srinivas <davanum@gmail.com>
