Provide a way to only download dependencies

[emanspeaks]

While developing my changes for this PR for utf8, I struggled with the fact that the GitHub Actions workflow was trying to download and build the library and utpp test framework using CPM. The problem with this, of course, is that relying on getting the package via git clone with a hardcoded URL in the GitHub action precludes effective testing for feature branches in forked repos. While it's probably possible to use GitHub variables to derive the correct URL and branch/commit to use with CPM, it's probably just easier to use the standard GitHub action to checkout the commit in question locally and operate on that. However, it seems that there is not a simple way to have CPM download dependencies given a local version of the base package. I resorted to having CPM download the packages in a separate location and then copy in utpp into my working area after checking out the commit with the standard GitHub action.

It's possible this is really an artifact of my lack of experience using CPM, so if there's a way to accomplish what I was trying to do already in a simple way, feel free to close this issue.