config: Revert removal of {packages}/{opts} hack

This was removed in e4d0d60, and replaced with a
neater approach, however the newer approach doesnt
work for replacements from other sections.

Fixes tox-dev#1777

docs/changelog/1777.bugfix.rst

@@ -0,0 +1 @@
+Fix regression that broke using install_command in config replacements - by :user:`jayvdb`

src/tox/config/__init__.py

@@ -1848,6 +1848,12 @@ def _replace_match(self, match):
             return os.pathsep
 
         default_value = g["default_value"]
+        # special case: opts and packages. Leave {opts} and
+        # {packages} intact, they are replaced manually in
+        # _venv.VirtualEnv.run_install_command.
+        if sub_value in ("opts", "packages"):
+            return "{{{}}}".format(sub_value)
+
         if sub_value == "posargs":
             return self.reader.getposargs(default_value)