[9.x] Add wherehas soft deleting scopes

[tiagof]

An attempt to fix/Improve mergeConstraintsFrom to use correct qualified column names.

Related issue --> #42075

[BrandonSurowiec]

Would match the code style of the rest of the tests (by using $this over self:: and inline your variables $versionsA and $versionsT. More like this:

    public function testSelfReferencingRelationshipWithSoftDeletes()
    {
        [$taylor, $abigail] = $this->createUsers();

        $this->assertCount(1, $abigail->versions);
        $this->assertTrue($abigail->versions->first()->is($taylor));

        $this->assertCount(0, $taylor->versions);
        $this->assertEquals(1, SoftDeletesTestUser::whereHas('versions')->count());
    }

There also may be a better name than version for this "test" relationship. Maybe self_referencing() to make the test relation match the test case, making it more clear?

    public function testSelfReferencingRelationshipWithSoftDeletes()
    {
        [$taylor, $abigail] = $this->createUsers();

        $this->assertCount(1, $abigail->self_referencing);
        $this->assertTrue($abigail->self_referencing->first()->is($taylor));

        $this->assertCount(0, $taylor->self_referencing);
        $this->assertEquals(1, SoftDeletesTestUser::whereHas('self_referencing')->count());
    }

    public function self_referencing()
    {
        return $this->hasMany(SoftDeletesTestUser::class, 'user_id')->onlyTrashed();
    }

[tiagof]

Agree and done :)