Don't install everything to the same prefix

[mwoehlke-kitware]

After spending some time fighting with KWIVER's broken Python bindings (n.b. Kitware/kwiver#1278), I had a thought. We ought to consider changing, at least as an option (which CI should use), how we build fletch to use a separate install prefix for every package.

This would provide isolation, so that e.g. a project that wants to use one package from fletch can do so without risking "contamination" from other packages also build by fletch (such as in the case of a developer using one fletch build for multiple projects) that the project doesn't want. This would also help catch problems like the aforementioned issue where a project is missing includes for some package, but this is hidden due to including some other package. Finally, this would greatly simplify cleaning a build of a particular package, since it would not be necessary to nuke the installs of every fletch package in order to clean out the remnants of a single package.

[dstoup]

I think this is a great idea! It will certainly take a lot of thought and planning but is a worthy task. I will start thinking about it and try putting together a simple WIP PR as proof of concept.