CLN: move/reorg pandas.tools -> pandas.core.reshape
xref #13634
> Could we, internally, just import from pandas.core.reshape ? Instead of from pandas.core.reshape.reshape, pandas.core.reshape.pivot, ..) Makes the imports somewhat shorter, and also makes it easier to change things within the reshape submodule without having to change all imports

yes this is a bit tricky as I have to elevate the the (all internal) API from the sub-modules to ``reshape`` not a big deal though. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=h1) Report
> Merging [#16032](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e0cbc37ccc02c3ee539a19d9043b541ace0d2733?src=pr&el=desc) will **decrease** coverage by `0.23%`.
> The diff coverage is `94.02%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16032/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16032      +/-   ##
==========================================
- Coverage   90.99%   90.76%   -0.24%     
==========================================
  Files         153      155       +2     
  Lines       50469    50470       +1     
==========================================
- Hits        45924    45808     -116     
- Misses       4545     4662     +117
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.52% <94.02%> (-0.24%)` | :arrow_down: |
| #single | `40.42% <15.08%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `90.25% <ø> (ø)` | |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.62% <ø> (ø)` | |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `0% <0%> (-93.69%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `81.87% <0%> (ø)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `95.08% <100%> (ø)` | |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.27% <100%> (ø)` | |
| [pandas/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL19faW5pdF9fLnB5) | `92.3% <100%> (-0.88%)` | :arrow_down: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `87.69% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.65% <100%> (ø)` | :arrow_up: |
| ... and [23 more](https://codecov.io/gh/pandas-dev/pandas/pull/16032?src=pr&el=tree-more) | |

------

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