WIP: deepcopy-gen: hack to ensure directory name is set

[justinsb]

When we use --input-dirs ./... deepcopy-gen / defaulter-gen cannot determine the package name, it appears (unless GOPATH is set).

Instead use the fully-qualified path as we're using for other generators.

../<dirname> also works but is not consistent with our other generators.

[justinsb]

Let's not merge yet, I'm trying to figure out all the intricacies here!