ACME: Remove dependency on luci-lib-ipkg

Instead, use @jow-'s suggestion of just checking for the presence of the
executables to find the installed web servers.

Fixes #8529.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

net/acme/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=acme
 PKG_VERSION:=2.7.9
-PKG_RELEASE:=8
+PKG_RELEASE:=9
 PKG_LICENSE:=GPLv3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz

net/acme/files/acme-cbi.lua

@@ -10,11 +10,10 @@ Copyright 2016 Toke Høiland-Jørgensen <toke@toke.dk>
 
 ]]--
 
-local ipkg = require "luci.model.ipkg"
+local fs = require "nixio.fs"
 
-local nginx_presence = ipkg.installed("nginx-all-module") or ipkg.installed("nginx-ssl") or false
-
-local uhttpd_presence = ipkg.installed("uhttpd") or false
+local nginx_presence = fs.access("/usr/sbin/nginx") or false
+local uhttpd_presence = fs.access("/usr/sbin/uhttpd") or false
 
 m = Map("acme", translate("ACME certificates"),
 	translate("This configures ACME (Letsencrypt) automatic certificate installation. " ..