Validate template variables in dependencies section during lint

[carolynvs]

When a bundle that uses advanced dependencies is built, the linter should check that all template variables used in the dependencies section of porter.yaml are in our list of available template variables.

We support the following variables:

• bundle.*
• installation.*
• outputs.OUTPUT_NAME only from the context of a dependency's defined output mappings.

Example:

parameters:
- name: stuff

credentials:
- name: token

outputs:
- name: connstr
  
dependencies:
  requires:
  - name: mysql
    parameters:
      dbstuff: ${bundle.parameters.stuff}
    credentials:
      token: ${bundle.credentials.token}
    outputs:
      connstr: https://${outputs.user}:${outputs.password}@${bundle.parameters.host}:${outputs.port}

In the example above, we would check that the stuff parameter and the token credential are defined on the parent bundle. We would also check that the mysql dependency has an output named port, user and password, and that the parent bundle has a parameter named host.

New outputs variable

The outputs variable is new and is a shorthand for accessing an existing variable, bundle.dependencies.CURRENT_DEP.outputs.OUTPUT_NAME.

Each output mapped for a dependency must be evaluated separately because ${outputs.OUTPUT_NAME} can only be validated in the context of the dependency in which it is defined.

ℹ️ Read PEP003 - Advanced Dependencies for context about how dependencies should work, design details, and notes about desired behavior.