Implement _tz_convert_tzlocal_utc for de-duplication
There is a bunch of code in conversion (and a bit in resolution, period, and tslib) that can be de-duplicated.  This is one part of ~5 steps in that process.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21727?src=pr&el=h1) Report
> Merging [#21727](https://codecov.io/gh/pandas-dev/pandas/pull/21727?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62a0ebc5837a4a1f7cc925e13344be1adf1acc59?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21727/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21727?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21727   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files         158      158           
  Lines       49705    49705           
=======================================
  Hits        45693    45693           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.99% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21727?src=pr&el=footer). Last update [62a0ebc...6878a69](https://codecov.io/gh/pandas-dev/pandas/pull/21727?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Yes, the private function is just for this module.