[kbenson]

By "a way to force invalidation of that cache" I didn't mean automatic invalidation, I meant a way to flag that you want it to re-download dependencies and store them for later use. I'm not sure where you got the requirement that it needs to automatically determined by a computer from my comment. I was thinking the "cache" could be as simple as the person setting up the build environment downloading the dependencies and configuring the build to use them. That's a local cache, when discussing automatic downloading of dependencies during building.

Set up your build environment with whatever manual intervention is required so that it can run without downloading remote resources. Build as needed. There is no reason for, and many reasons against, downloading dependencies during the build process, but that doesn't necessitate duplicating those dependencies within your own source tree. As long as there are directions on how to download a specific, definitive version of the dependency, whether that is automated or not isn't really a big deal if it's done infrequently.