meson: drop three more single-use convenience libraries

meson: drop three more single-use convenience libraries

The way that the cryptsetup plugins were built was unnecessarilly complicated.
We would build three static libraries that would then be linked into dynamic
libraries. No need to do this.

While at it, let's use a convenience library to avoid compiling the shared code
more than once.

We want the output .so files to be located in the main build directory,
like with all consumable build artifacts, so we need to maintain the split
between src/cryptsetup/cryptsetup-token/meson.build and the main meson.build
file.

AFAICT, the build artifacts are the same: exported and undefined symbols are
identical. There is a tiny difference in size, but I think it might be caused
by a different build directory name.