test_packages_distributions_all_module_types: Create valid import names

The import names that were created by these tests were not valid Python
identifiers.

Fix that, and furthermore: add another check to verify that _all_ import
names returned from packages_distributions() are always valid Python
identifiers. Ideally we should check that all keys returned from
packages_distributions() are valid import names (i.e. can be imported),
but this is at least a step in the right direction.

tests/test_main.py

@@ -342,10 +342,10 @@ def test_packages_distributions_all_module_types(self):
         filenames = list(
             itertools.chain.from_iterable(
                 [
-                    f'{i}-top-level{suffix}',
-                    f'{i}-in-namespace/mod{suffix}',
-                    f'{i}-in-package/__init__.py',
-                    f'{i}-in-package/mod{suffix}',
+                    f'top_level_{i}{suffix}',
+                    f'in_namespace_{i}/mod{suffix}',
+                    f'in_package_{i}/__init__.py',
+                    f'in_package_{i}/mod{suffix}',
                 ]
                 for i, suffix in enumerate(suffixes)
             )
@@ -367,9 +367,14 @@ def test_packages_distributions_all_module_types(self):
         distributions = packages_distributions()
 
         for i in range(len(suffixes)):
-            assert distributions[f'{i}-top-level'] == ['all_distributions']
-            assert distributions[f'{i}-in-namespace'] == ['all_distributions']
-            assert distributions[f'{i}-in-package'] == ['all_distributions']
+            assert distributions[f'top_level_{i}'] == ['all_distributions']
+            assert distributions[f'in_namespace_{i}'] == ['all_distributions']
+            assert distributions[f'in_package_{i}'] == ['all_distributions']
+
+        # All keys returned from packages_distributions() should be valid import
+        # names, which means that they must _at least_ be valid identifiers:
+        for import_name in distributions.keys():
+            assert import_name.isidentifier(), import_name
 
 
 class PackagesDistributionsEggTest(