Fast skip dependencies

[artiomn]

One of the CPM problem - slowing down the build on the developer's machine.
Even if I didn't edit CMakeLists, CPM is checking dependencies.

But usually dependencies doesn't change often.

Maybe it's possible to skip full-checking dependencies, if they was already downloaded and nothing changed in the description?
And use add_subdirectory() only to add dependencies?

Probably this is implemented already, but not in the obvious way.