meson: define PROJECT_VERSION as the "bare" project version
Let's not use atoi() if we can simply provide the project version as a number.
In C code, this is the numerical project version. In substitutions in other
files, this is just the bare substitution.
The "PACKAGE_" prefix is from autotools, and is strange. We call systemd a
"project", and "package" is something that distros build. Let's rename.
PACKAGE_URL is renamed to PROJECT_URL for the same reasons and for consistency.
(This leave PACKAGE_VERSION as the stringified define for C code.)