Replace manual opam file with template

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

dune.opam

@@ -1,27 +1,23 @@
-opam-version: "2.0"
-maintainer: "opensource@janestreet.com"
-authors: ["Jane Street Group, LLC <opensource@janestreet.com>"]
-homepage: "https://github.com/ocaml/dune"
-bug-reports: "https://github.com/ocaml/dune/issues"
-dev-repo: "git+https://github.com/ocaml/dune.git"
-license: "MIT"
-depends: [
-  "ocaml" {>= "4.02"}
-  "base-unix"
-  "base-threads"
-]
 build: [
   # opam 2 sets OPAM_SWITCH_PREFIX, so we don't need a hardcoded path
   ["ocaml" "configure.ml" "--libdir" lib] {opam-version < "2"}
   ["ocaml" "bootstrap.ml"]
   ["./boot.exe" "--release" "--subst"] {pinned}
   ["./boot.exe" "--release" "-j" jobs]
 ]
+opam-version: "2.0"
+dev-repo: "git+https://github.com/ocaml/dune.git"
+authors: ["Jane Street Group, LLC <opensource@janestreet.com>"]
+license: "MIT"
 conflicts: [
   "jbuilder" {!= "transition"}
   "odoc" {< "1.3.0"}
 ]
-synopsis: "Fast, portable and opinionated build system"
+depends: [
+  "ocaml" {>= "4.02"}
+  "base-unix"
+  "base-threads"
+]
 description: """
 dune is a build system that was designed to simplify the release of
 Jane Street packages. It reads metadata from "dune" files following a
@@ -40,3 +36,4 @@ several opam roots/switches simultaneously. This helps maintaining
 packages across several versions of OCaml and gives cross-compilation
 for free.
 """
+synopsis: "Fast, portable and opinionated build system"

dune.opam.template

@@ -0,0 +1,7 @@
+build: [
+  # opam 2 sets OPAM_SWITCH_PREFIX, so we don't need a hardcoded path
+  ["ocaml" "configure.ml" "--libdir" lib] {opam-version < "2"}
+  ["ocaml" "bootstrap.ml"]
+  ["./boot.exe" "--release" "--subst"] {pinned}
+  ["./boot.exe" "--release" "-j" jobs]
+]