Restrict base/build-base combinations

[mr-cal]

What needs to get done

Per ST119, bases and build-bases need restrictions including:

1. base: coreXX build-base: coreYY is not allowed
2. base: coreXX build-base: devel is only allowed when coreXX is unstable
3. base: devel is not allowed

See the table in ST119 for a full definition of valid and invalid combinations.

The BASES_ constants and get_effective_base() need to be reworked. For example, devel is considered a valid base.

Why it needs to get done

To prevent snaps with undefined build and runtime behavior.