Restructure validate command to use SchemaDirectory

[willosborne]

Feature Request

Description of Problem:

Currently the validate command uses its own logic to load schemas. This is custom, poorly tested and also can make remote URL calls to fetch non-file paths which may present a security issue.

This depends on #830

Potential Solutions:

Suggested approach is to remove the logic in the loadMetaSchemas() method and instead have that initialise the SchemaDirectory.

Then the loadSchema callback when initialising ajv will just defer to the SchemaDirectory, rather than loading any files.
That way the directory has control of what is loaded, and also any error handling logic.

ajv will still handle pulling references out - so only the schema loading features of the directory are needed.