dvc: refactor package versioning logic

- Use `version.py` to compute a dynamic version stored in `__version__`.
  Version information was scattered across the source code
  (`VERSION_BASE`, `VERSION`, `__version__`, and `version.py`).

- Read `__version__` from `setup.py` and **pin** it using custom
  `build_py` command class.
  This way, we can maintain consistancy between the build version
  and the runtime one.
  Also, this gives us the ability to run `setup.py` without depending on
  on `dvc` itself.

Close #1824

.gitignore

@@ -24,8 +24,6 @@ innosetup/config.ini
 
 .coverage
 
-dvc/version.py
-
 *.swp
 
 pip-wheel-metadata/