REF/CLN: Move private method
Moved these two internal function to the only location where they are used in the codebase.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=h1) Report
> Merging [#24875](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12bb6d0536b947f74b66ebe18b5f451bfdd7453f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24875/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24875      +/-   ##
==========================================
+ Coverage   92.38%   92.39%   +<.01%     
==========================================
  Files         166      166              
  Lines       52412    52411       -1     
==========================================
  Hits        48423    48423              
+ Misses       3989     3988       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.88% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `88.68% <100%> (+0.11%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.13% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24875?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/24875?src=pr&el=footer). Last update [12bb6d0...cdcbfbc](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=h1) Report
> Merging [#24875](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12bb6d0536b947f74b66ebe18b5f451bfdd7453f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24875/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24875      +/-   ##
==========================================
+ Coverage   92.38%   92.39%   +<.01%     
==========================================
  Files         166      166              
  Lines       52412    52410       -2     
==========================================
- Hits        48423    48422       -1     
+ Misses       3989     3988       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.88% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/core/computation/expr.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXhwci5weQ==) | `88.68% <100%> (+0.11%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24875/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.13% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24875?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/24875?src=pr&el=footer). Last update [12bb6d0...8508dc8](https://codecov.io/gh/pandas-dev/pandas/pull/24875?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. thanks @mroeschke 