Better logic for editing mapped items

Related to #997, but a more general requirement for Embedded Git with IPM.

- If an item is not mapped, follow source control rules for current namespace (top level or IPM-based)
- If an item is mapped, and the IPM package containing it exists in the namespace default routine database for some other namespace and was loaded in dev mode, edits should follow that other namespace's source control settings
- If an item is mapped and in a non-dev mode IPM package, don't allow it to be touched.

Start by reviewing related issue #997 and tracing the existing Embedded Git and IPM source-control decision path. Done means unmapped items use the current namespace rules, mapped development packages use the owning namespace's settings, and mapped non-development packages cannot be edited.

Written by the indexing model from the issue text.