[SPARK-12988][SQL] Can't drop top level columns that contain dots

## What changes were proposed in this pull request?

Fixes "Can't drop top level columns that contain dots".

This work is based on dilipbiswal's https://github.com/apache/spark/pull/10943.
This PR fixes problems like:

```
scala> Seq((1, 2)).toDF("a.b", "a.c").drop("a.b")
org.apache.spark.sql.AnalysisException: cannot resolve '`a.c`' given input columns: [a.b, a.c];
```

`drop(columnName)` can only be used to drop top level column, so, we should parse the column name literally WITHOUT interpreting dot "."

We should also NOT interpret back tick "`", otherwise it is hard to understand what

```
​```aaa```bbb``
```

actually means.

## How was this patch tested?

Unit tests.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13306 from clockfly/fix_drop_column.