BLD: rename some extensions
xref #12588 

This is a move (and deprecation of json/parser) of ``pandas.json``, ``pandas.parser``,

also moves some extensions into the main structure as appropriate

renames extensions to follow the pattern "libxxx" for consistency (as well as some renaming of the .pyx files themselves). In effect locates the library code next to the calling code.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=h1) Report
> Merging [#15532](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d0a281fd60a2099c932151280af88d5392ea9a84?src=pr&el=desc) will **decrease** coverage by `-0.04%`.
> The diff coverage is `92.59%`.


```diff
@@            Coverage Diff             @@
##           master   #15532      +/-   ##
==========================================
- Coverage   91.06%   91.03%   -0.04%     
==========================================
  Files         136      139       +3     
  Lines       49099    49120      +21     
==========================================
+ Hits        44714    44718       +4     
- Misses       4385     4402      +17
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/msgpack/_version.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2lvL21zZ3BhY2svX3ZlcnNpb24ucHk=) | `100% <ø> (ø)` | |
| [pandas/io/msgpack/exceptions.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2lvL21zZ3BhY2svZXhjZXB0aW9ucy5weQ==) | `83.33% <ø> (ø)` | |
| [pandas/compat/pickle_compat.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `68.29% <ø> (ø)` | :white_check_mark: |
| [pandas/json.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2pzb24ucHk=) | `100% <100%> (ø)` | |
| [pandas/io/msgpack/__init__.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2lvL21zZ3BhY2svX19pbml0X18ucHk=) | `79.16% <100%> (ø)` | |
| [pandas/core/reshape.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS5weQ==) | `99.25% <100%> (ø)` | :white_check_mark: |
| [pandas/io/sas/__init__.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9fX2luaXRfXy5weQ==) | `100% <100%> (ø)` | |
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `79.64% <100%> (ø)` | :white_check_mark: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.06% <100%> (ø)` | :white_check_mark: |
| [pandas/__init__.py](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?src=pr&el=tree#diff-cGFuZGFzL19faW5pdF9fLnB5) | `92.3% <100%> (+0.64%)` | :white_check_mark: |
| ... and [19 more](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15532?src=pr&el=footer). Last update [d0a281f...afa606f](https://codecov.io/gh/pandas-dev/pandas/compare/d0a281fd60a2099c932151280af88d5392ea9a84...afa606fce20376cc7069aa460ebce4722ce8a9f1?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). closing in favor of #15537 which does it all.