correlated-agg in group by can't be detected and optimized & groupBy building should be after selection

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
create table t(a int, b int)
tidb> explain select 1 from t n where b>0 group by (select count(n.b) from t);
```

### 2. What did you expect to see? (Required)
correlated-agg should be evaluated outside, and be linked into its sub-query.

the main reason is that:
`resolveGroup` build the correlated-agg in the sub-query directly. (plan tree has changed after that)

MySQL is special: it allows sub-query in where and group clause, that's why we need to analyze these two before we really build them

we need to split this functionality into two parts:

1: we need a group clause analysis process
2: we need a building group process after the selection (selection should be built before groupby)

### 3. What did you see instead (Required)
```
+----------------------------------+----------+-----------+---------------+-------------------------------------------------+
| id | estRows | task | access object | operator info |
+----------------------------------+----------+-----------+---------------+-------------------------------------------------+
| Projection_10 | 3333.33 | root | | 1->Column#8 |
| └─HashAgg_11 | 3333.33 | root | | group by:Column#7, funcs:firstrow(1)->Column#11 |
| └─Apply_13 | 3333.33 | root | | CARTESIAN left outer join |
| ├─TableReader_16(Build) | 3333.33 | root | | data:Selection_15 |
| │ └─Selection_15 | 3333.33 | cop[tikv] | | gt(test.t.b, 0) |
| │ └─TableFullScan_14 | 10000.00 | cop[tikv] | table:n | keep order:false, stats:pseudo |
| └─HashAgg_17(Probe) | 1.00 | root | | funcs:count(test.t.b)->Column#7 |
| └─TableReader_23 | 10000.00 | root | | data:TableFullScan_22 |
| └─TableFullScan_22 | 10000.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+----------------------------------+----------+-----------+---------------+-------------------------------------------------+
```

### 4. What is your TiDB version? (Required)

master