Sub-libraries allow a single-level hierarchy inside a symbol library.
This is useful for database libraries and for supporting other EDA library types where a
single file can contain multiple logical groupings of symbols.
Allows placing parts from an external database that reference symbols from another loaded library.
Includes:
- nanodbc wrapper
- database schematic library plugin
- basic tests
Fixes https://gitlab.com/kicad/code/kicad/-/issues/7436
Jon Evans