MAINT: Make pandasSQL_builder private
Per discussion with @jorisvandenbossche <a href="https://github.com/pydata/pandas/pull/13611#discussion_r71048508">here</a>.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13668?src=pr) is **84.38%**

> Merging [#13668](https://codecov.io/gh/pydata/pandas/pull/13668?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13668   diff @@
==========================================
  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43223      43223          
  Misses         8000       8000          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d7c028d...8183e04](https://codecov.io/gh/pydata/pandas/compare/d7c028d4965932160fa3b69f56c716b1454c42a5...8183e04d63319dd3c67a5b32ca2feeb9ebf98028?src=pr)
 @sinhrks , @jorisvandenbossche , @jreback : any updates on this?  Seems relatively trivial to merge.
 lgtm. @jorisvandenbossche 
 Sorry for the delay. 
I did a quick search on github, and few people did actually use this in their code (and that were not copies of pandas), so not sure this is worth to change. 
I meant we should have made it internal when it was added in 0.14, but not sure if it is worth to change.
 @jorisvandenbossche : Ah, that's a good point.  If the intention was internal, but people are actually using this function for their own `pandas` engines, I wonder if we should even make it internal at all.  It seems like it could be useful for people's own custom usages then.
 Yep, then we just let it be :-)
 Sorry for my inclarity triggering you to action
 No worries.  Good that we at least investigated.
