Create Artifact abstraction class

Background

For Artifact as a Proto an abstract Artifact class, which will implement artifact specific functions which currently reside in Element.

Task description

• Create list of existing functions that should be abstracted out of Element via Artifact

Current list of potential artifact related funcs to abstract:

• __compute_splits (imports files dir of artifact)

• stage_artifact (imports files dir of artifact)

• _stage_sources_at (checks if a requested buildtree is cached empty)

• _cache_artifact

• _get_build_log

• _cached_buildtree

• __extract

• __get_artifact_metadata_keys

• __get_artifact_metadata_dependencies

• __get_artifact_metadata_workspaced

• __get_artifact_metadata_workspaced_dependencies

• __load_public_data

• __load_build_result

• Abstract out, no new tests to be added

Acceptance Criteria

Current bst functionality is preserved

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