workspaces via sourcecache 1

introduce a 'workspace' kind source plugin and stage workspaces via the sourcecache

work towards #985 (closed)

• workspaces via sourcecache 1/2:

  • append 'workspace' source during element collection and store the element sources. Forbid the workspace plugin manually.
  • add minimal 'workspace' source plugin
  • reset workspaces using the original sources held by the workspace source object
  • remove additional handling for workspaces in functions responsible for loading/staging.

• remove workspace cache clearing (#1088 (closed))

• remove workspace handling in element cache key calculation (partial reversion of #1073 (closed))

• fix buildbox issues (from strict xfails passing)

• clean history

• re-benchmark against master

Benchmark (on a wsl dev machine - sequential runs)

====show====
master time: 5.77
branch time: 5.79
...which is a +0.35% difference

master memory: 170.0
branch memory: 170.0
...which is a -0.0% difference

====build-4====
master time: 645.99
branch time: 636.84
...which is a -1.42% difference

master memory: 196.0
branch memory: 196.0
...which is a -0.0% difference

====build-8====
master time: 639.67
branch time: 635.93
...which is a -0.58% difference

master memory: 196.0
branch memory: 196.0
...which is a -0.0% difference

====build-12====
master time: 621.34
branch time: 623.12
...which is a +0.29% difference

master memory: 196.0
branch memory: 196.0
...which is a -0.0% difference

====show-cached====
master time: 12.61
branch time: 15.19
...which is a +20.46% difference

master memory: 199.0
branch memory: 200.0
...which is a +0.5% difference