BLD: rename some extensions

[jreback]

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-io]

Codecov Report

Merging #15532 into master will decrease coverage by -0.04%.
The diff coverage is 92.59%.

@@ 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	Coverage Δ
pandas/io/msgpack/_version.py	100% <ø> (ø)
pandas/io/msgpack/exceptions.py	83.33% <ø> (ø)
pandas/compat/pickle_compat.py	68.29% <ø> (ø)	✅
pandas/json.py	100% <100%> (ø)
pandas/io/msgpack/init.py	79.16% <100%> (ø)
pandas/core/reshape.py	99.25% <100%> (ø)	✅
pandas/io/sas/init.py	100% <100%> (ø)
pandas/io/excel.py	79.64% <100%> (ø)	✅
pandas/util/testing.py	82.06% <100%> (ø)	✅
pandas/init.py	92.3% <100%> (+0.64%)	✅
... and 19 more

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[jreback]

closing in favor of #15537 which does it all.