PTFM-13152 Remove app-specific validations when building applets.

Summary:
Additionally check for existence of applet with same name in
destination before uploading resources when "dx build" not
supplied with "-f" or "-a" flag and add tests for building applets.

Remove do_build_step and do_upload_step from build_and_upload_locally() declaration.

Add lint fixes and minor changes.

Test Plan: ./test_dxclient.py TestDXBuildApp

Reviewers: psung

Reviewed By: psung

Subscribers: psung

Differential Revision: https://phabricator.dnanexus.com/D149