Only add merged IAM policies for Lambda when they will be used (#6262)

[onebytegone]

What did you implement:

Closes #6262

How did you implement it:

Made serverless only add the wildcard IAM policy for canonically named functions if there are actually canonically named functions in the service.

How can we verify it:

Full source: log-group-policy.zip

service: log-group-policy

provider:
   name: aws
   runtime: nodejs8.10

functions:
   testFunction:
      name: ${self:service}-custom-named-fn
      handler: index.handler

mkdir /tmp/test-packaging-for-6262
cd /tmp/test-packaging-for-6262
unzip log-group-policy.zip
cd log-group-policy
echo '{}' > package.json
npm i serverless@1.44.1
./node_modules/.bin/sls package
cp .serverless/cloudformation-template-update-stack.json cf-update-stack-v1.44.1.json
${PATH_TO_SERVERLESS_CLONE}/bin/serverless package
cp .serverless/cloudformation-template-update-stack.json cf-update-stack-using-PR.json
diff cf-update-stack-v1.44.1.json cf-update-stack-using-PR.json

The above is based on tests from #6262.

Expected: There should be no change to the IAM policies for the example stack.

Todos:

Note: Run npm run test-ci to run all validation checks on proposed changes

• Write tests and confirm existing functionality is not broken.
  Validate via npm test
• Write documentation
  • Does this need to be documented? If so, where does it belong?
• Ensure there are no lint errors.
  Validate via npm run lint-updated
  Note: Some reported issues can be automatically fixed by running npm run lint:fix
• Ensure introduced changes match Prettier formatting.
  Validate via npm run prettier-check-updated
  Note: All reported issues can be automatically fixed by running npm run prettify-updated
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[onebytegone]

⬆️ rebase was to pull in the CI fix from #6535. Hopefully the tests will pass now...

[medikoo]

Looks great. Thank you @onebytegone !