Enable Deppy to be imported by other packages

[oceanc80]

Move all interfaces and broader implementations of those interfaces from internal to pkg
Only keep internal implementation details of the interfaces in internal

Make Deppy importable

• Keep solver package in internal
• Export sat/constraints.go and sat/variable.go

[akihikokuroda]

How is the solver triggered by keeping the solver package in internal? I think the 'sat' package should be internal but the solver package may need to be exported.

[perdasilva]

Yeah, I agree. It might be hard to keep the sat package internal though. The reason is the constraint interface relies has package internal methods that are required by the solver. Furthermore, those methods make use of package internal structures (e.g. litMapping). For now, I think we should move everything to pkg to make it easy to be imported by other package. Then move things that are internal to the implementation to internal. This may require some refactoring of the sat solver package though.

[perdasilva]

I've put up the PR to move everything to pkg: #79

[akihikokuroda]

It may be difficult to achieve #50 once we expose the sat package. Are we giving it up?