Unify charm build planner with default build planner

[mr-cal]

The charm build planner was written before the default build planner. To reduce duplication, they should be consolidated.

Differences:

• default build planner allows an implicit build-for from the platform name
• charm build planner does not require a single element build plan if build-for: [all] is specified
  • charmcraft validates all is only used in one place
• charm build planner allows for multi-base builds