add default registry if none provided during pkg install prompt (#2862)

Author: IanButterworth

src/REPLMode/REPLMode.jl

@@ -655,6 +655,10 @@ const help = gen_help()
 
 function try_prompt_pkg_add(pkgs::Vector{Symbol})
     ctx = Context()
+    if isempty(ctx.registries)
+        Registry.download_default_registries(ctx.io)
+        ctx = Context()
+    end
     available_uuids = [Types.registered_uuids(ctx.registries, String(pkg)) for pkg in pkgs] # vector of vectors
     available_pkgs = pkgs[isempty.(available_uuids) .== false]
     isempty(available_pkgs) && return false