[SPARK-14993][SQL] Fix Partition Discovery Inconsistency when Input is a Path to Parquet File

#### What changes were proposed in this pull request?
When we load a dataset, if we set the path to ```/path/a=1```, we will not take `a` as the partitioning column. However, if we set the path to ```/path/a=1/file.parquet```, we take `a` as the partitioning column and it shows up in the schema.

This PR is to fix the behavior inconsistency issue.

The base path contains a set of paths that are considered as the base dirs of the input datasets. The partitioning discovery logic will make sure it will stop when it reaches any base path.

By default, the paths of the dataset provided by users will be base paths. Below are three typical cases,
**Case 1**```sqlContext.read.parquet("/path/something=true/")```: the base path will be
`/path/something=true/`, and the returned DataFrame will not contain a column of `something`.
**Case 2**```sqlContext.read.parquet("/path/something=true/a.parquet")```: the base path will be
still `/path/something=true/`, and the returned DataFrame will also not contain a column of
`something`.
**Case 3**```sqlContext.read.parquet("/path/")```: the base path will be `/path/`, and the returned
DataFrame will have the column of `something`.

Users also can override the basePath by setting `basePath` in the options to pass the new base
path to the data source. For example,
```sqlContext.read.option("basePath", "/path/").parquet("/path/something=true/")```,
and the returned DataFrame will have the column of `something`.

The related PRs:
- https://github.com/apache/spark/pull/9651
- https://github.com/apache/spark/pull/10211

#### How was this patch tested?
Added a couple of test cases

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

Closes #12828 from gatorsmile/readPartitionedTable.