Move TraceInfo to a new place

[aiuto]

There are a few reasons for this:

• TraceInfo is a debugging hack internal to the metadata gathering rules. It does not belong with the broad facing rules. The rules to declare licenses and their kinds should be in a distinct bazel package from the ones that consume them. #85
• There is often a need for organizations to provide local implementations of gathering rules. Thus, we may have two copies of gather_metadata.bzl (and/or licenses_core.bzl) in our source tree.
• But having two distinct labels for a provider causes immense import pain.

Further cleanups to separate gather_* from rules to rules_gathering are coming.