extend source api and remove private use from workspace plugin

closes #1161 (closed) requires !1649 (merged)

• replace stage_into_directory with plugin implementations
• prevent double call to _stage via sourcecache
• remove dependence on _get_local_path from
• add test: WorkspaceSource.stage() should only be called once
• benchmark (investigated some slowdown)

====show====
master_results time: 6.33
traveltissues_1161_results time: 6.39
which is a +0.95% difference
master_results memory: 171.0
traveltissues_1161_results memory: 171.0
which is a -0.0% difference
====build-4====
master_results time: 704.75
traveltissues_1161_results time: 695.04
which is a -1.38% difference
master_results memory: 197.0
traveltissues_1161_results memory: 197.0
which is a -0.0% difference
====build-8====
master_results time: 702.66
traveltissues_1161_results time: 697.77
which is a -0.7% difference
master_results memory: 197.0
traveltissues_1161_results memory: 197.0
which is a -0.0% difference
====build-12====
master_results time: 685.98
traveltissues_1161_results time: 682.04
which is a -0.57% difference
master_results memory: 197.0
traveltissues_1161_results memory: 197.0
which is a -0.0% difference
====show cached====
master_results time: 14.01
traveltissues_1161_results time: 14.13
which is a +0.86% difference
master_results memory: 199.0
traveltissues_1161_results memory: 199.0
which is a -0.0% difference

• use new flag in local plugin (this is non-trivial) [!1669 (closed)]
• squash commits
• fix update script issue #1183 (closed)