This will allow us to use its content from other languages than Lua as
well.

PKG_FILE_DEPENDS caused the whole site repo to be hashed to determine if
gluon-site needed rebuilding. While this is normally no problem,
alternative build setups sometimes put the Gluon repo inside the site repo,
causing long build times and parallel build failures.

/lib/gluon/release had been lost during the conversion to the new site config
format.