Rearrange pulses module into a subpackage

[alecandido]

As the title says.

I'm trying to make small and self-contained PRs to simplify the review process.

Btw, notice that this is non-breaking (or it could be, unless I'm missing something), since everything is still exposed as before. Nonetheless, this is on top of all the other simplifications, so it will be part of 0.2 in any case (trying to land it soon in main).

[codecov[bot]]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (7b43744) 61.73% compared to head (a5b7248) 61.94%.

Files	Patch %	Lines
src/qibolab/pulses/sequence.py	94.65%	7 Missing :warning:
src/qibolab/pulses/pulse.py	94.44%	4 Missing :warning:
src/qibolab/pulses/plot.py	96.59%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## simplify-pulse-6 #769 +/- ## ==================================================== + Coverage 61.73% 61.94% +0.20% ==================================================== Files 47 52 +5 Lines 5721 5723 +2 ==================================================== + Hits 3532 3545 +13 + Misses 2189 2178 -11 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibolab/pull/769/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibolab/pull/769/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `61.94% <95.56%> (+0.20%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

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[alecandido]

In this specific PR it is actually increasing (+0.2%), even though I acknowledge that the Codecov message is sometimes confusing:

Comparison is base (810ee62) 61.63% compared to head (271230c) 61.84%.

base (first) is the base branch, the one I'm sitting on top, and head (second) is the latest commit of this PR.

In general, for the other PRs, the trick is that I'm deleting code. If you check the hidden table Misses lines are usually decreasing, but also the overall lines (even covered ones), so the percentage may change, and decrease as well (usually it decreases, since most of the uncovered lines are in instruments, that I'm not touching). But yeah, for Qibolab coverage is a difficult topic, and for refactoring the patch statistics is not that meaningful...

[alecandido]

It is kind of a small change, but I knew you could appreciate ;)