VDR: Refactor types and didservice packages

[reinkrul]

Replaces #2471

Goals of this PR are:

• Getting rid of the types package, since packages should imply function, not technical contents
• Getting rid of the didservice package, since it's a dumping place for every "service"-y type that doesn't fit anywhere else. And the same argument as for types applies here as well.
• Making a clear separation of management (create, update, lower-level stuff) of DIDs owned by the current node and resolving any DID/key/service (regardless whether it's owned by the node or not).

Changes:

• Removed vdr/didservice/common.go#CompoundService (type alias for map[string]string), was just used in didman.go
• Removed vdr/didservice/common.go#ErrDIDAlreadyExists (unused)
• Removed vdr/didservice/test.go (unused)
• Moved all did-resolving-related types (constants, interfaces, implementations) from vdr/types and vdr/didservice to vdr/resolvers/did.go. The same for keys and services.
• Renamed vdr/didservice#KeyResolver implementation to DIDKeyResolver (otherwise it clashes with the interface name).
• Renamed vdr/didservice#ServiceResolver implementation to DIDServiceResolver (otherwise it clashes with the interface name).
• Moved all DID creation/updating types in vdr/didservice to vdr/management/management.go
• Moved all DID "finding" types (iterating over Nuts DID store) to vdr/management/finder.go
• Renamed VDR main struct from VDR to Module, to avoid clash with the interface name (later on, the VDR interface should be renamed as well, since packages shouldn't contain types which starts with the same word or are named the same). We could do this for all module implementations, for consistency.

No functionality or behavior has been changed/removed.

[woutslakhorst]

Is there a reason to have a separate management and resolver package?

What happens if you place contents of those packages in the VDR root?

[reinkrul]

Is there a reason to have a separate management and resolver package?

Separation of concerns; management is for everything you want to do with your own DIDs, resolver is for resolving any DID (or key, or service) for any DID method. We could dump it in 1 common package, but then we end up with a service package again.

What happens if you place contents of those packages in the VDR root?

Cyclic dependencies with at least network module.