Support packaging cairo-plugins in Scarb

[maciektr]

1. Handle packaging of builtin = true Cairo plugins - only manifest normalization
2. Change package manifest normalization to include cairo-plugin section
3. Allow packaging if target=lib or cairo-plugin (not only lib)
4. Make sure verification runs for proc-macro plugins and builtin plugins
5. (maybe design doc?) Do we need to package rust code with cargo package first? Do we need Cargo.toml manifest normalization?