Removing libarchive as a separate submodule

Rather than trying a lot of cmake magic, we will provide
instructions on how to build and set up libarchive in external/libarchive.
CMake: Building libarchive outside this project, added Find*.cmake
scripts.

It turned out to be too tricky to build libarchive with a dependency
on zlib with CMake (including support for older versions, etc.)
so we now shift to a build where static builds of libarchive and zlib
are conducted in advance.