The plugins cache now contains plugins file path relative to the base
plugins directory (the directory that contains plugins.dat). The cache
can hence be generated from a different absolute directory than the one
it is later installed to.

For native or emulated native builds, this enables build-time cache
generation.

Note however that the relative paths are different in the build tree
(modules/*/.libs/lib*_plugin.*) and in standard installation
(plugins/*/lib*_plugin.*). The cache cannot be copied from the build
tree to the installation directory.