codewide: re-structure modules

[wprzytula]

According to the new proposed design, the structure of modules and imports is changed to provide more rational code separation.

• scylla-cql is no longer exposed; only some entities are selectively re-exported using new cql module;
• transport module is decomposed and a number of new modules emerge instead.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• [ ] I added appropriate Fixes: annotations to PR description.

[Lorak-mmk]

v2: #817