Finalize JSON output & add JSON schema

[wagoodman]

Finalizes the json output format:

• fixes the image.layers to populate layer content
• pretty prints the json output --this does make the output larger, but more comprehensible for diffing and groking snapshots and output in general.
• renames source -> directory
• adds struct tags to defend output against variable name refactors (which should not impact the schema)
• enhances (and fixes) APK files output to include more metadata

In general this PR adds:

• a JSON schema to validate the JSON presenter output and act as a contract for external users to build tooling against
• integration tests which generate json output examples from the existing package cases, and validates the JSON presenter output against the JSON schema
• a generate.py script and generate-json-schema make target for easily updating the json schema from integration test output

Future PR:

• add detection of presenter additions that could be incorporated into the schema, ensuring the schema is as up to date as possible.

Closes #105
Closes #104