change to libapt-pkg abi 5.0 with versioned symbols

change to libapt-pkg abi 5.0 with versioned symbols

We changed an aweful lot of stuff, so 5.0 is properly better than 4.X as
a semantic version and as we are at it lets add some trivial symbol
versioning as well: We just mark all exported symbols with the same
version for now. This isn't really the proper thing to do as if we add
symbols in later versions (with the same abi) they will get the same
symbols version, but our .symbols file will protect us from the problems
arising from this as it will ensure that a package acutally depends on a
version of the abi high enough to include the symbol.