Fixes for install root and plugin detection.

[jglogan]

• Sets up API server as source of truth for installation root, similarly to what was done for the data root. system start establishes the install root, setting the environment variable CONTAINER_INSTALL_ROOT when launching the API server.
• The API server propagates the environment variable when launching helpers, and returns the install root to the CLI via the health check XPC.
• Includes several fixes for detecting plugins that use app bundle layout.