Optional creation of buildtrees

Description

Provide configuration for the optional creation of buildtrees

Artifacts can be cached explicitly with an empty build tree when built via the cli main options or user config for all or only successful build artifacts. Default behaviour is to still create and cache all expected buildtrees.

This will create a situation in which a cache-key could contain a ref to a buildtree dir that was or wasn't populated for relevant element types and as such this uncertainty should be opt-in behaviour until further work such as artifact has a proto is landed. Users will be warned if interacting with an empty buildtree.

Changes proposed in this merge request:

• Add main cli and user config option cache-buildtrees as a 'tristate' for when artifacts are cached via bst build
• Keep default behaviour the same, highlight were a cached buildtree is empty
• Tests

This merge request, when approved, will close: #896 (closed)

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