Don't self-reference the x_package.go's own package in import proxy's argument types

[WilliamNHarvey]

Fixes a bug in genimport with proxy package types.

In my example, before we would write

package williamnharveyfrontend
...
DoSomethingWithInterfaceStruct_	func(interface{}, context.Context, *williamnharveyfrontend.InterfaceStruct) (*williamnharveyfrontend.InterfaceStruct, error)

But after we correctly write

package williamnharveyfrontend
...
DoSomethingWithInterfaceStruct_	func(interface{}, context.Context, *InterfaceStruct) (*InterfaceStruct, error)

This frontend/backend pattern is common in golang apps that use stubbing for tests or for optional ways to implement a common backend

Happy to rip out the example and only merge the actual fix upstream if that's preferred. I didn't see an existing test otherwise