Metastore code refactor for clearer naming and flow

[dafeder]

Proposal to tighten up the naming and typing conventions in the metastore module. Some changes are simply renaming or separating existing classes/methods, but others change the types being passed around. The current code is difficult to understand and maintain.

This assumes that the metastore module has been merged into the dkan module (see #3746).

Drupal\dkan\Metastore\Item

Classes and interfaces for defining Metastore item objects. These will be the primary metadata objects passed around the system.

Class	Description
Drupal\dkan\Metastore\Item\MetastoreItemInterface	Interface for a metastore item object
Drupal\dkan\Metastore\Item\MetastoreEntityItemInterface	Extension interface for entity-based items
Drupal\dkan\Metastore\Item\MetastoreNodeItem	Metastore item class based on a node.

Drupal\dkan\Metastore\Schema

Classes for defining, retrieving and validating against metastore schemas. Note that SchemaManager replaces the old SchemaRetriever service. Assumes completion of #3761.

Class	Description
Drupal\dkan\Metastore\Schema\MetadataValidator	Utility class for validating metadata against a schema in the system.
Drupal\dkan\Metastore\Schema\MetastoreSchema	Schema plugin definition, for schemas that will be defined in YAML.
Drupal\dkan\Metastore\Schema\MetastoreSchemaInterface	Interface for schema plugins.
Drupal\dkan\Metastore\Schema\SchemaManager	Plugin manager to gather definitions and retrieve schemas.