We already have a proper message when the creation of an alias fails
because of a name conflict. Also add a message when the primary filename
of a domain config is already occupied by another domain's alias.

Also add an 'Error:' prefix to the existing message to easier to see.

The old bash-based parsing code was way too complex. Replace it with Lua.

When two domains alias the same name (or one aliases another), display a
meaningful error message like:

    Failed to alias domain 'foo' as 'bar', name already taken by
    domain 'baz'.

Solves a recursive dependency problem.

While we're at it, also fix the description string.

Based-on-patch-by: lemoer <git@irrelefant.net>

The domain configs are not checked yet, and not used for anything.

Based-on-patch-by: lemoer <git@irrelefant.net>

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.