Cache Key Dictionary Changes

Background

https://mail.gnome.org/archives/buildstream-list/2019-July/msg00004.html

Formalize a public specification for the cache key dictionary

The element.py cache key dictionary should have a formal public specification to ensure maximal key value/performance.

Tasks

• Make field changes summarised at list
  • remove context
  • remove project
  • remove cache
  • rename artifact-version to core-artifact-version
  • rename execution-environment to sandbox
• Replace element with element-plugin-key, element-plugin-name, and element-plugin-version.
• Split dependencies into dependency-keys-strong and dependency-keys-weak
• Replace sources with a list of dictionaries of name, version, and key. This may be overwritten by the values of workspaces which is removed.