Fix base_build() with a proper support for multi-package-building through conda-build

This MR fixes build.base_build() to properly handle building multiple versions of the same package according to what is defined in conda_build_config.yaml.

conda/conda_build_config.yaml

@@ -22,6 +22,3 @@ python:
   - 3.6
   - 3.7
   - 3.8
-
-setuptools:
-  - 46.1.1