orm: forward-port join-order fix from v9.2.1 to v10

[OscarYuen]

Summary

83b941300e97 ("Change join order") landed on the v9 branch and shipped in v9.2.1 (2021-04-22). It was never forward-ported to v10, so every v10 release — up to and including v10.15.1 — still has the pre-fix ordering.

This applies the same change to v10.

What the ordering does

In manyQuery and m2mQuery, the user-supplied ApplyQuery hook runs immediately after q.Model(...):

q = q.Model(manyModel)
if j.ApplyQuery != nil {
    q, err = j.ApplyQuery(q)   // <- runs here
}
if len(q.columns) == 0 {
    q.columns = append(q.columns, &hasManyColumnsAppender{j})
}

baseTable := j.BaseModel.Table()
// ... "FK IN (parent ids)" appended here, plus the polymorphic condition

So a callback passed as:

db.Model(&users).Relation("Posts", func(q *orm.Query) (*orm.Query, error) {
    return q.Where("..."), nil
}).Select()

sees a query that has not yet been scoped to the parent rows, and the
len(q.columns) == 0 check that decides whether to add the default
has-many columns is evaluated at the same early point.

After this change both run at the end of the function, once the relation
query is fully constructed — matching v9.2.1.

Diff

Pure code motion in orm/join.go: the ApplyQuery block and the
adjacent default-column block move from just after q.Model(...) to just
before return q, nil, in both manyQuery and m2mQuery. No behaviour
is added or removed beyond the ordering. 24 insertions, 22 deletions, one
file.

Verification

• go build ./... clean
• go test ./orm/... ./types/... ./internal/... — all pass
• gofmt clean
• Resulting manyQuery / m2mQuery ordering is identical to v9@v9.2.1

I don't have a Postgres instance wired up here, so the DB-backed
integration tests were not exercised — worth a CI run.