Expand esy command expressions in pesy config

[ManasJayanth]

Using the trick used to figure out library path of an imported library, we could also determine other variables like #{@opam/zarith.lib}, #{@opam/zarith.etc} etc - paths in the install tree of a given package.

If needed, paths in the source tree can be looked up in _esy/installation.json

This will allow pesy configs like

{
  "flags": ["-ccopt -L#{esy-zlib.lib} -lz"]
}