fix: add certain raw imports to RESERVED_NAMES (#824)

The current example is 'auth', which is imported directly by both
transports and unit tests. This name conflicts with any calculated
dependency import whose name happens to be 'auth'.
Fix involves adding 'auth' and other direct, raw imports in templates
to RESERVED_NAMES.

gapic/utils/reserved_names.py

@@ -21,7 +21,15 @@
 # They are explicitly allowed message, module, and field names.
 RESERVED_NAMES = frozenset(
     itertools.chain(
+        # We CANNOT make exceptions for keywords.
         keyword.kwlist,
+        # We make SOME exceptions for certain names that collide with builtins.
         set(dir(builtins)) - {"filter", "map", "id", "input", "property"},
+        # This is a hand-maintained list of modules that are directly imported
+        # in templates, i.e. they are not added as dependencies to any type,
+        # the raw text is just there in the template.
+        # More can be added as collisions are discovered.
+        # See issue #819 for additional info.
+        {"auth", "credentials", "exceptions", "future", "options", "policy", "math"}
     )
 )