patch action should fail without source stanza

[Alizter]

For packages without a source stanza in their lock file, the patch action makes no sense and leads to errors. Therefore we should give an informative error for the user in this case.

@ElectreAAS demonstrated this by omitting a the source field from an opam file causing Dune to fail in a peculiar way:

$ dune build
Error:
_build/.sandbox/65c0a64cb0715bb9bd484397e646319c/_private/default/.pkg/ocamlbuild.0.16.1+dune-7e71f9afe87a779ea2a759bd9ab31127/source/Makefile:
No such file or directory
-> required by
   _build/_private/default/.pkg/ocamlbuild.0.16.1+dune-7e71f9afe87a779ea2a759bd9ab31127/target/cookie