Better manage search paths in ProjBuilder

Hi,

I finally managed to build `proj` on Windows. 🎉

When bundling and distributing `proj.db` from the `.exe` the way I found seems to be to use `ProjBuilder` and `set_search_paths`. Which only appends one path to the internal search paths.

This is a problem as there is no way to clear the search paths or reordering them to prioritise my distributed `proj.db` path. Instead it will always be the last place to look at.

Instead it would be better to have proper `get` and `set` functions so reordering or clearing of the search paths can be done manually.

Or is there another way of doing this I have overseen?

Locate ProjBuilder and its set_search_paths implementation, then trace how internal search paths are stored and consulted. The work is done when callers can retrieve, replace, clear, and reorder those paths so a distributed proj.db can be prioritized; check existing tests around ProjBuilder if present.

Written by the indexing model from the issue text.