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

scripts/build_posix.sh

@@ -42,7 +42,7 @@ command_exists()
 fpm_build()
 {
 	print_info "Building $1..."
-	VERSION=$(python -c "import dvc; from dvc import VERSION; print(str(VERSION))")
+	VERSION=$(python -c "import dvc; from dvc import __version__; print(str(__version__))")
 	fpm -s dir \
 	    -f \
 	    -t $1 \

scripts/innosetup/config_gen.py

@@ -1,10 +1,10 @@
 # This script generates config.ini for setup.iss script
-from dvc import VERSION
+from dvc import __version__
 from dvc.utils.compat import ConfigParser
 
 config = ConfigParser.ConfigParser()
 config.add_section("Version")
-config.set("Version", "Version", VERSION)
+config.set("Version", "Version", __version__)
 
 with open("scripts/innosetup/config.ini", "w") as f:
     config.write(f)