Now that we require cmake 3.15.0 or better, we can use the
`$<TARGET_OBJECTS:...>` generator for any type of library, not just
object libraries.  This means we do not need to create an unnecessary
abstraction in an object library.

Removing this unnecessary target greatly simplifies the DX for IDE
users.

Upgrade our cmake requirements so that we can adopt bigger, better
features.

cmake: further refactorings

The select hashes module selects the hash; the CMakeLists.txt selects
the files to implement it.

Update the global variables `LIBGIT2_OBJECTS` to
`LIBGIT2_DEPENDENCY_OBJECTS` for clarity and consistency.

Apply the standard project cmake formatting to the modules.