types: allow EntityQuery to select from many tables

[tantaman]

Doing join will bring in more tables that can be selected against. Thus, EntityQuery.select needs to be able pull from more than a single schema.

To do that, EntityQuery now takes a FromSet rather than EntitySchema.

type FromSet = {
  [table: string]: EntitySchema
};

const q = new EntityQuery<{issue: Issue}>();
const q2 = new EntityQuery<{user: User}>();

q.join(q2, 'owner', 'issue.ownerId', 'owner.id').select(...);

join requires us to support aliasing ambiguous columns so this adds that as well.

// aliasing: x as y
foo.select(['x', 'y'])
// qualifying
foo.select('foo.bar', 'baz.x').join(Baz, 'baz_id', 'id');
// unqualified works too if the column is not ambiguous
foo.select('bar');
// added support for *
foo.select('issue.*')

[tantaman]

Merged with @arv 's latest changes around complex where conditions.

[arv]

LGTM