Move GuiceSlickProvider to an internal package

[octonato]

This PR #1360 exposed the fact that all Guice modules are got impacted by the MiMa checks while we should consider them internal API.

It also exposed that com.lightbend.lagom.javadsl.persistence.jdbc.GuiceSlickProvider is a public API. It's a Guice provider, thus only for internal wiring, and it provides the SlickProvider that itself is a private class.

I suggest that all Guice modules are moved to com.lightbend.lagom.internal (or a subpackage of it) and that the Provider become internal and private.

WDYT?
/cc @ignasi35 @TimMoore

[TimMoore]

It's intentional that the module classes are public, even though we don't expect people to have a binary dependency on them, they might use the names to enable or disable modules.

See @jroper's comment: #1360 (comment)

These modules are not designed to be used directly by users, they're only there so that users can explicitly disable them using play.modules.disabled.

Changing the package structure would break existing config, and it would mean that these don't appear in the Java API documentation, so people won't know how to disable them without reading the Lagom source code.

I agree though that GuiceSlickProvider should become internal.