Allow ListCommand to choose tool by version

This adds the function `ListCommand.fetch_tool_by_simple_spec`. This
allows the caller to specify a SimpleSpec and the name of a tool, and
the function will return the Updates.xml record for the specified tool
with the largest version number that satisfies the SimpleSpec. This is
meant to allow users to install tools without specifying an explicit
version number.

This also adds some test code to validate the complicated parts of this
code.

This also extends `helper.xml_to_modules`, so that a caller can
automatically get all tags in an XML element without explicitly
specifying them.

tests/test_helper.py

@@ -1,8 +1,10 @@
 import binascii
 import os
 
+import pytest
 import requests
 from requests.models import Response
+from semantic_version import Version
 
 from aqt import helper
 
@@ -95,3 +97,16 @@ def test_helper_downloadBinary_sha256(tmp_path, monkeypatch):
     helper.downloadBinaryFile(
         "http://example.com/test.xml", out, "sha256", expected, 60
     )
+
+
+@pytest.mark.parametrize(
+    "version, expect",
+    [
+        ('1.33.1', Version('1.33.1')),
+        ('1.33.1-202102101246', Version('1.33.1-202102101246')),
+        ('1.33-202102101246', Version('1.33.0-202102101246')),
+        ('2020-05-19-1', Version('2020.0.0-05-19-1')),
+    ],
+)
+def test_helper_to_version_permissive(version, expect):
+    assert helper.to_version_permissive(version) == expect