We introduce two paths where we can hard code search prefixes
(`/additional-search-prefixes`) and additional binaries paths
(`/share/revng/additional-bin-paths`).
This enables us to add to `additional-bin-paths` the configure-time
specified path of the preferred LLVM installation, instead of relying on
`PATH` to look up, e.g., `opt` at run-time.
`revng.api` now autodetects files (libraries, pipeline yamls) for
initialization via the same mechanism used by `revng.cli`. This removes
the need to pass them via environment variables from `revng daemon`
and allows easy interaction with python's C API.