[C++][Parquet] Organize src code file structure to have a very clear folder with public headers.

[asfimport]

We should organize the source code structure to have a folder where all the public headers are and nothing else. This makes it easy to understand what is the public API and which APIs needed to be looked at wrt to compatibility.

Reporter: Nong Li / @nongli
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as PARQUET-482. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Effectively parquet/parquet.h is the only public header right now. How about having parquet/api/ contain the public headers (or parquet/api.h if it only needs to be one file)?

[asfimport]

Wes McKinney / @wesm:
patch for this: apache/parquet-cpp#70

[asfimport]

Julien Le Dem / @julienledem:
Issue resolved by pull request 70
apache/parquet-cpp#70