Combine dkan and metastore modules

[dafeder]

Continuing the work laid out in #3746, we do not need separate a module for "metastore". There is no DKAN w/o the metastore. One #4412 is complete, merge "metastore" into "dkan" as well.

Acceptance criteria

• Metastore services can maintain the same name. It is fine for them to be prefixed with dkan.metastore.
• As appropriate, move Drupal\metastore namespaces into Drupal\dkan root or Drupal\dkan\Metastore. More discussion needed to determine what goes where here.
• Of course, upgrade path. Table names should not change but there will likely be some update hook magic needed for this to be smooth. Documentation for custom code integrations refactor a must.

More discussion needed to finalize sequence of merging/release here. Should this and #4412 merge into a 3.x branch that we maintain in parallel? Can this be released under 2.x as it doesn't technically add or rename any modules?