Use a consistent user-level cache directory instead of installation-relative paths

Currently, BVS stores its data relative to the installation directory. With one installation this is fine, but with multiple installations it can lead to multiple data directories scattered across the machine. This makes disk space management more challenging as there can be multiple, large data directories present on the machine in different places, rather than one well-known place.

Related to #640

Start by locating where BVS derives its installation-relative data path, then read the context in related issue #640. Done means all installations use one consistent user-level cache directory instead of scattering data across installation locations.

Written by the indexing model from the issue text.