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.
When revng cli was invoked with a bare namespace, e.g. `revng ptml`
it would produce an ugly exception.
Fix this behavior by instead rerunning the command with `--help`
appended.
We used to collect all binaries into the `bin/` directory. However this
led to confusions since certain commands where available both as
`revng-command` and `revng command`.
This commit moves all the executables except `revng` into
`libexec/revng`, which, according to FHS, is dedicated to "internal
binaries that are not intended to be executed directly by users or shell
scripts".