Add placeholder package extensions for JuliaInterface/OscarInterface

PackageInfo.g

@@ -430,6 +430,15 @@ Dependencies := rec(
 
   ExternalConditions := []),
 
+# These extensions are essentially empty placeholders for now, see
+# https://github.com/semigroups/Semigroups/issues/1220 for more details.
+Extensions := [
+  rec(needed := [["JuliaInterface", ">=0.9"]],
+      filename := "gap/julia.g"),
+  rec(needed := [["OscarInterface", ">=1.3.0"]],
+      filename := "gap/oscar.g"),
+],
+
 BannerString := Concatenation(
   "----------------------------------------------------------------------",
   "-------\n",

gap/julia.g

@@ -0,0 +1,22 @@
+#############################################################################
+##
+##  julia.g
+##
+##  Licensing information can be found in the README file of this package.
+##
+#############################################################################
+##
+##  This file is a GAP package extension (see the "Extensions" component of
+##  PackageInfo.g): it is only read if the GAP package JuliaInterface is
+##  available. It is currently a placeholder, and does not yet do anything.
+##
+##  The long term aim is to use this extension mechanism to call the
+##  functionality of libsemigroups from GAP via the Julia package
+##  Semigroups.jl (https://github.com/libsemigroups/Semigroups.jl), as a
+##  replacement for (parts of) the Semigroups kernel module. See
+##  https://github.com/semigroups/Semigroups/issues/1220 for more details.
+##
+##  The JuliaInterface itself can be found at
+##  https://github.com/oscar-system/GAP.jl/tree/master/pkg/JuliaInterface
+
+SEMIGROUPS.JuliaInterfaceLoaded := true;

gap/oscar.g

@@ -0,0 +1,19 @@
+#############################################################################
+##
+##  oscar.g
+##
+##  Licensing information can be found in the README file of this package.
+##
+#############################################################################
+##
+##  This file is a GAP package extension (see the "Extensions" component of
+##  PackageInfo.g): it is only read if the GAP package OscarInterface is
+##  available. It is currently a placeholder, and does not yet do anything.
+##
+##  See gap/julia.g, and https://github.com/semigroups/Semigroups/issues/1220
+##  for more details of why this extension exists.
+##
+##  The OscarInterface itself can be found at
+##  https://github.com/oscar-system/Oscar.jl/tree/master/gap/pkg/OscarInterface
+
+SEMIGROUPS.OscarInterfaceLoaded := true;