Decouple xlrd reading from ExcelFile class
To support community engagement on #11499 I figured it was worth refactoring the existing code for read_excel which very tightly couples xlrd. 

There are quite a few ways to go about this but here I am creating a separate private reader class for the xlrd engine which gets dispatched to for parsing and metadata inspection. In theory a similar class could be made for openpyxl.

I didn't make a base class here to keep diff minimal though I certainly could if we see a need for it
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=h1) Report
> Merging [#24423](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/159772d0c9d49977d4b7f871c62c26e06890143a?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24423/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24423      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51950    51950              
==========================================
+ Hits        47953    47954       +1     
+ Misses       3997     3996       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24423/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24423?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/24423?src=pr&el=footer). Last update [159772d...d015b12](https://codecov.io/gh/pandas-dev/pandas/pull/24423?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/24423?src=pr&el=h1) Report
> Merging [#24423](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/befd32495f5ffdc83bc7b76e8f79ecaacbc441ce?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24423/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24423      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51950    51954       +4     
==========================================
+ Hits        47953    47957       +4     
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/24423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9iYXNlLnB5) | `91.83% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.91% <0%> (ø)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/24423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.68% <0%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24423?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/24423?src=pr&el=footer). Last update [befd324...607dbe5](https://codecov.io/gh/pandas-dev/pandas/pull/24423?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Latest commit should have changes. In looking at the test cases I think there also needs to be a decoupling done there to really support this (ex: something like `test_read_from_http_url` should exist in the base class, not just TestXlrdReader).

Can take a stab here or do in separate PR if it makes the diff easier. Lmk @WillAyd might as well do it here. Opened #24472 as a logical follow up thanks @WillAyd 