Automatic dependency resolution

[skhrshin]

I'm trying to create a tool for automatic dependency resolution between packages using CPM, including support of diamond-shape dependencies. I pushed the first experimental version: Dapper. If you have an interest, please take a look.

Right now, it is doing a hacky thing which depends on the implementation detail of CPM when it passes the resolved dependencies to CPM: call CPMDeclarePackage and then propagate CPM_DECLARATION_${PACKAGE_NAME} to the parent scope. Also, it directly references CPM_SOURCE_CACHE in order to determine the directories to clone repositories into. (I tried CPMAddPackage first, but it locked the package version to the first version fetched due to FetchContent, so I switched to git clone.) Not only these points, there are many issues and possibilities of improvements as it is still experimental.