[swift5][client] Fix module and class with same name

[4brunu]

It fixes #9953

With #9624 we changed a class from #projectName#API to #projectName#.

But it seems that the Swift compiler had a bug related to projects that have a class with the same name of the module.

More details on this #9953 and https://bugs.swift.org/browse/SR-898.

This PR reverts the change from #projectName#API to #projectName#, but keeps the reference to a warning to help with the transition.
Once the transition is made, this warning can be removed with the new introduced flag removeMigrationProjectNameClass.

@aymanbagabas do you have other suggestions to fix this issue?

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[aymanbagabas]

@4brunu no, I think this workaround is the best way to solve this issue at least until the bug is fixed.