fix: remove auth, policy, and options from the reserved names list

[busunkim96]

Fixes #835.

Breakdown by name that was originally added in #824

• auth: from google import auth -> import google.auth
• credentials: from google.auth import credentials -> from google.auth import credentials as ga_credentials
• exceptions: from google.api_core import exceptions -> from google.api_core import exceptions as core_exceptions
• future: skipped, as it is only used in the generated tests and has a low chance of colliding
• options from google.iam.v1 import options_pb2 as options -> from google.iam.v1 import options_pb2
• policy from google.iam.v1 import policy_pb2 as policy -> from google.iam.v1 import policy_pb2
• math skipped as it is only used in generated tests

For options and policy there is a small change to gapic/schema/metadata.py to not alias _pb2 types

[codecov]

Codecov Report

Merging #851 (0443936) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #851   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1655   +47     
  Branches       328       338   +10     
=========================================
+ Hits          1608      1655   +47     

Impacted Files	Coverage Δ
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
gapic/utils/checks.py	100.00% <100.00%> (ø)
gapic/utils/options.py	100.00% <100.00%> (ø)

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