debian/*: make it possible to co-install -dev package

The pkg-config is renamed to ofono-ubports.pc, and headers are put in a subdirectory. The plugin directory is also varianted, which mean the plugin built against 1 variant will be used with that variant only.

Bug: ofono-apndb-plugin#1 (closed)
See: ubports/development/core/packaging/ofono-sailfish!16 (merged)