Use $schema and $id in the right way

[jaimergp]

Description

Closes #282

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

xref pydantic/pydantic#1478

[jaimergp]

I'm not sure about the migration path for this change. Need to take a look at the validation code in conda-build, but with this change it looks like we could just use JSON schema directly without having menuinst around.

We should also publish the schema to jsonstore.

Questions to address:

• How do we version the schema? I've seen several options:
  • Independent versioning with SchemaVer or regular SemVer. Right now we just have the schema, but it will keep evolving as we are seeing in CEP-11: Update schema with menuinst 2.1.x and 2.2.x changes ceps#98.
  • Same as menuinst? I don't like this, should be tied to a tool agnostic method.
• How do we name the schema filename once we decide on a version scheme?
• Do we keep all schema versions around in the repo?
• Think of runtime validation.

[marcoesters]

I'm not sure about the migration path for this change. Need to take a look at the validation code in conda-build, but with this change it looks like we could just use JSON schema directly without having menuinst around.

We should also publish the schema to jsonstore.

Questions to address:

* How do we version the schema? I've seen several options:
  
  * Independent versioning with SchemaVer or regular SemVer. Right now we just have _the_ schema, but it will keep evolving as we are seeing in [CEP-11: Update schema with menuinst 2.1.x and 2.2.x changes ceps#98](https://github.com/conda/ceps/pull/98).
  * Same as menuinst? I don't like this, should be tied to a tool agnostic method.

* How do we name the schema filename once we decide on a version scheme?

* Do we keep all schema versions around in the repo?

* Think of runtime validation.

This should probably be an issue to discuss this more, but I think schema version and menuinst version should be independent. The major versions will probably coincide though.

I don't know much about SchemaVer, but if this is commonly used, I don't see why we wouldn't use that one for the schema. For the file name, I would then prefer something like menuinst.schema.<SchemaVer>.json since SchemaVer uses dashes.