New analyzer: detect missing backend component

[sympatheticmoose]

If a plugin specifies backend=true and/or alerting=true we should check for the existence of a backend binary.

Should fail If backend: true + no default executable (need to have at least one file starting with gpx_ ) OR if executable property set -> at least one file starting with whatever is in this field.

Should warn if alerting=true, but backend=false or is not specified