Add system-packages-ensure

Also an alias for system-packages-package-installed-p to
executable-find.  This is somewhat related to bug #26, since you can
now do something clever with if statments: if pack-man=apt, then
ensure blah, else if pack-man=pacman, ensure blah-foo.

Obviously, there's still work to be done here.

system-packages.el

@@ -353,6 +353,22 @@ manger."
   (interactive "sPackage to install: ")
   (system-packages--run-command 'install pack args))
 
+;;;###autoload
+(defun system-packages-ensure (pack &optional args)
+  "Ensure PACK is installed on system.
+Search for PACK with `system-packages-package-installed-p', and
+install the package if not found.  Use ARGS to pass options to
+the package manager."
+  (if (system-packages-package-installed-p pack)
+      t
+    (system-packages-install pack args)))
+
+;;;###autoload
+(defalias 'system-packages-package-installed-p #'executable-find
+  "Return t if PACK is installed.
+Currently an alias for `executable-find', so it will give wrong
+results if the package and executable names are different.")
+
 ;;;###autoload
 (defun system-packages-search (pack &optional args)
   "Search for system packages.