handle duplicate packages passed to installer steps

Currently, the build will fail if, e.g., the user passes `numpy==x.y.z` to the `PIP_PACKAGES` `build-arg`, and `numpy==a.b.c` was hard-coded to be pip-installed in that step.

The solution would probably be to split to-be-installed packages into an array, find duplicates based on substrings before `=`, and prioritize to the user's version. That amount of logic shared across images would likely require writing a bash function in a separate file, adding it to the build context for `cdl-base`, and `COPY`ing it in during the `cdl-base` build. Unfortunately, there are currently no `COPY` steps in the current `cdl-base` build, so this would mean adding an a new layer to the base image and increasing the size of it _and_ all child images.

Difference may be de minimis, but not entirely clear. Will probably end up writing this either way and checking the `docker image history` for `cdl-base` images build with and without this addition to decide if it's worth it.