Bugfix: respect order of mirrors in mirrors.yaml (#13544)

Commands like "spack mirror list" were displaying mirrors in a
different order than what was listed in the corresponding mirrors.yaml
file.

This restores commands to iterate over mirrors in the order that
they appear in the config file.

lib/spack/spack/mirror.py

@@ -21,6 +21,8 @@
 
 import ruamel.yaml.error as yaml_error
 
+from ordereddict_backport import OrderedDict
+
 try:
     from collections.abc import Mapping
 except ImportError:
@@ -166,7 +168,7 @@ class MirrorCollection(Mapping):
     """A mapping of mirror names to mirrors."""
 
     def __init__(self, mirrors=None, scope=None):
-        self._mirrors = dict(
+        self._mirrors = OrderedDict(
             (name, Mirror.from_dict(mirror, name))
             for name, mirror in (
                 mirrors.items() if mirrors is not None else
@@ -178,6 +180,7 @@ def to_json(self, stream=None):
     def to_yaml(self, stream=None):
         return syaml.dump(self.to_dict(True), stream)
 
+    # TODO: this isn't called anywhere
     @staticmethod
     def from_yaml(stream, name=None):
         try: