prepare-release: Also search for libraries in CMake locations

prepare-release: Also search for libraries in CMake locations

With this change, the 'library' command looks for a library libX
in the directories: build/bin, */X, X.

This allows it to find the library when building with the
upcoming CMake backend, which places the libraries in a sub
directory of the build tree with the same name as the source
tree.

For example, if building in 'build/', the apt-pkg library
will be available at 'build/apt-pkg/libapt-pkg.so.5.0'.

In case there are multiple instances of a library,
the newest one will be used.

Gbp-Dch: ignore