Implement a parser for tools, and instantiate once for
arglib-netencode arguments (parsed by the new netencode parser) and
one just from the PATH for testing from the repl.
Change-Id: Id0cf264100123a87700880c7230d68426224fd0d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7798
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI