Create implicit alias `it` only if there is no explicit alias

[larsk21]

When there is a single source class in a query, the implicit alias it is created to refer to instances of the source class in the target section of the query. When the developer instead assigns an explicit alias, we should not create the implicit alias it to avoid confusion or conflicts.

Example:

from Film
create {
    it.name // usage of implicit alias `it`
}

from Film f
create {
    f.name // `it` should not be available here
}