refact: support absolute paths in local source manager

[masci]

This PR introduced the concept that the path for a service with a local source is relative to the deployment file. For example, if you have a deployment file /foo/bar/deployment.yml containing the following:

services:
  dummy_workflow:
    name: Dummy Workflow
    source:
      type: local
      name: src  # Relative to what?
    path: workflow:echo_workflow

the local source manager will look for src in /foo/bar/src.

This change required a refactoring of the source managers:

• managers now take a DeploymentConfig instance in the constructor, hence the protocol was changed into an abstract class
• the deployment config has a new field base_path that's automatically populated when from_yaml is used.

[coveralls]

coverage: 79.167% (+0.3%) from 78.854%
when pulling 7a712b6 on massi/source_manager_refact
into f625bfd on main.

[logan-markewich]

Seems good to me, sensible change 👍🏻 I agree by default it should be relative to the yml

Main worry is that maybe we can auto-detect relative vs. absolute paths

[masci]

Pasting here for visibility:

So to recap:

• /path/to/sources points to the absolute location
• path/to/sources refers to the folder containing the deployment file
• ./path/to/sources same as above