meson: define PROJECT_VERSION as the "bare" project version

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.)