Separate out internals.concat, internals.managers
Follow-up to #22014.

Moved `get_blockno_placements` to libinternals since a) its a natural fit anyway and b) it is needed in both concat and managers and I didn't want to runtime-import it.

The only non-cut/paste edit is a change of `isinstance(self.block, ExtensionBlock)` to `self.block.is_extension`

`__init__` namespace is big in part because `io.packers` uses `getattr(internals, name)` and is just really tightly intertwined with internals.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22028?src=pr&el=h1) Report
> Merging [#22028](https://codecov.io/gh/pandas-dev/pandas/pull/22028?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/716efd3f1e08d81743edcb7422fa5eb05523b58d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `96.88%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22028/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22028?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22028      +/-   ##
==========================================
+ Coverage   91.99%      92%   +<.01%     
==========================================
  Files         168      170       +2     
  Lines       50597    50601       +4     
==========================================
+ Hits        46549    46553       +4     
  Misses       4048     4048
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <96.88%> (ø)` | :arrow_up: |
| #single | `42.2% <68.1%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22028?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/22028/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL19faW5pdF9fLnB5) | `100% <100%> (+3.11%)` | :arrow_up: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/22028/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.52% <96.52%> (ø)` | |
| [pandas/core/internals/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/22028/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2NvbmNhdC5weQ==) | `98.37% <98.37%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22028?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/22028?src=pr&el=footer). Last update [716efd3...f7490a9](https://codecov.io/gh/pandas-dev/pandas/pull/22028?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 yep, thanks @jbrockmendel 