Read package flags from OPAM metadata

This changes the logic from hardcoding the compiler "base packages" to
instead read the OPAM flags to check whether the package in question is
a compiler in which case it will be considered a base package.

It also adds code to check for packages whether they are `conf` packages,
which could be used to check for virtual packages.

Inspired by tarides#327, this would allow `dkml-base-compiler` to be considered
a base package and not accidentally vendor it. However it doesn't yet
allow using the DKML compiler, because the name of the `ocaml` conf
package is still hardcoded with no way to override it (yet).

lib/config.ml

@@ -16,17 +16,7 @@
 open Import
 
 let base_packages =
-  [
-    "jbuilder";
-    "dune";
-    "ocamlbuild";
-    "ocamlmod";
-    "oasis";
-    "ocamlify";
-    "ocaml";
-    "ocaml-base-compiler";
-    "ocaml-variants";
-  ]
+  [ "jbuilder"; "dune"; "ocamlbuild"; "ocamlmod"; "oasis"; "ocamlify" ]
   |> List.map ~f:OpamPackage.Name.of_string
   |> OpamPackage.Name.Set.of_list
 

lib/opam.ml

@@ -174,25 +174,40 @@ module Pp = struct
   let url = Fmt.using OpamUrl.to_string Fmt.string
 end
 
+module Package_flag = struct
+  type t = OpamTypes.package_flag
+
+  let pp pps (v : t) =
+    match v with
+    | Pkgflag_LightUninstall -> Fmt.pf pps "light-uninstall"
+    | Pkgflag_Verbose -> Fmt.pf pps "verbose"
+    | Pkgflag_Plugin -> Fmt.pf pps "plugin"
+    | Pkgflag_Compiler -> Fmt.pf pps "compiler"
+    | Pkgflag_Conf -> Fmt.pf pps "conf"
+    | Pkgflag_AvoidVersion -> Fmt.pf pps "avoid-version"
+    | Pkgflag_Unknown unknown -> Fmt.pf pps "unknown(%s)" unknown
+end
+
 module Package_summary = struct
   type t = {
     package : OpamPackage.t;
     url_src : Url.t option;
     hashes : OpamHash.t list;
     dev_repo : string option;
     depexts : (OpamSysPkg.Set.t * OpamTypes.filter) list;
+    flags : Package_flag.t list;
   }
 
-  let pp fmt { package; url_src; hashes; dev_repo; depexts } =
+  let pp fmt { package; url_src; hashes; dev_repo; depexts; flags } =
     let open Pp_combinators.Ocaml in
     Format.fprintf fmt
       "@[<hov 2>{ name = %a;@ version = %a;@ url_src = %a;@ hashes = %a;@ \
-       dev_repo = %a;@ depexts = %a }@]"
+       dev_repo = %a;@ depexts = %a;@ flags = %a }@]"
       Pp.package_name package.name Pp.version package.version
       (option ~brackets:true Url.pp)
       url_src (list Hash.pp) hashes
       (option ~brackets:true string)
-      dev_repo Depexts.pp depexts
+      dev_repo Depexts.pp depexts (list Package_flag.pp) flags
 
   let from_opam package opam_file =
     let url_field = OpamFile.OPAM.url opam_file in
@@ -204,18 +219,25 @@ module Package_summary = struct
       Option.map ~f:OpamUrl.to_string (OpamFile.OPAM.dev_repo opam_file)
     in
     let depexts = OpamFile.OPAM.depexts opam_file in
-    { package; url_src; hashes; dev_repo; depexts }
+    let flags = OpamFile.OPAM.flags opam_file in
+    { package; url_src; hashes; dev_repo; depexts; flags }
+
+  let has_flag flag { flags; _ } = List.mem flag ~set:flags
+  let is_compiler v = has_flag OpamTypes.Pkgflag_Compiler v
 
   let is_virtual = function
     | { url_src = None; _ } -> true
     | { dev_repo = None | Some ""; _ } -> true
     | _ -> false
 
-  let is_base_package = function
-    | { package; _ }
-      when OpamPackage.Name.Set.mem package.name Config.base_packages ->
-        true
-    | _ -> false
+  let is_base_package v =
+    let in_base_pkgs { package; _ } =
+      OpamPackage.Name.Set.mem package.name Config.base_packages
+    in
+    let is_compiler_pkg { package; _ } =
+      OpamPackage.Name.equal package.name Config.compiler_package_name
+    in
+    is_compiler v || in_base_pkgs v || is_compiler_pkg v
 end
 
 module Dependency_entry = struct

lib/opam.mli

@@ -20,6 +20,7 @@ module Package_summary : sig
     hashes : OpamHash.t list;
     dev_repo : string option;
     depexts : (OpamSysPkg.Set.t * OpamTypes.filter) list;
+    flags : OpamTypes.package_flag list;
   }
 
   val pp : t Fmt.t

test/lib/test_duniverse.ml

@@ -25,7 +25,8 @@ let opam_factory ~name ~version =
 let summary_factory ?(name = "undefined") ?(version = "1") ?dev_repo ?url_src
     ?(hashes = []) ?(depexts = []) () =
   let package = opam_factory ~name ~version in
-  { Opam.Package_summary.package; dev_repo; url_src; hashes; depexts }
+  let flags = [] in
+  { Opam.Package_summary.package; dev_repo; url_src; hashes; depexts; flags }
 
 let dependency_factory ?(vendored = true) ?name ?version ?dev_repo ?url_src
     ?hashes ?depexts () =