Update `opam-admin` to infer the findlib file.

Read the arguments of `ocamlfind remove` to populate the PKG/findlib file.
We need a more robust function (which install the package) but that's already a
good start.

opam-repository$ opam-admin findlib --infer ocamlnet
Processing (package) ocamlnet.3.2.1
Processing (package) ocamlnet.3.5.1
Processing (package) ocamlnet.3.6.0
Processing (package) ocamlnet.3.6.3
Processing (package) ocamlnet.3.6.5
Processing (package) ocamlnet.3.7.3
Processing (package) ocamlnet.3.7.4
Processing (package) ocamlnet.3.7.4-1

opam-repository$ $ cat packages/ocamlnet/ocamlnet.3.7.4-1/findlib
equeue
equeue-gtk2
equeue-ssl
netcamlbox
netcgi2
netcgi2-plex
netclient
netgssapi
nethttpd
nethttpd-for-netcgi2
netmech-scram
netmulticore
netplex
netshm
netstring
netstring-pcre
netsys
netzip
pop
rpc
rpc-auth-local
rpc-generator
rpc-ssl
shell
smtp