`plugin_entry!` should specify the calling crate's package version

[jonas-schievink]

The plugin definition includes a plugin version, which we currently always set to 0.0.0. We should automatically set it to the plugin crate's Cargo package version instead.

The plugin_entry! macro can expand to cfg!("CARGO_PKG_VERSION_MAJOR") etc. to fetch the package version as a string, but afaik there's no crate that gives us the version as an integer, which we need for this.

[jonas-schievink]

Wrote a crate to fix this: https://github.com/jonas-schievink/pkg-version

Will publish it Soon™