add juliapkg.json to package data

[timmysilv]

Context: If you installed from a clone of the repo, you'll have juliapkg.json present. If you install from PyPI... not the case. This means that if a dependent project (eg. MrKite) tries to use MrMustard's Julia features, the Julia deps will not be auto-managed because this file won't exist in that venv!

Description of the Change: ~Add juliapkg.json to tool.setuptools.package-data so dependent projects use Julia correctly.~ (poetry did not respect the setuptools thing easily, and we don't need it because moving the file works)

Move juliapkg.json into the mrmustard/ folder so it's installed with mrmustard. Confirmed that juliacall finds it both with editable installs of MrMustard, and with it added to another project (eg. MrKite) as a dependency.

Benefits: Places that install MrMustard from PyPI can use the Julia features

Possible Drawbacks: N/A

Related GitHub Issues: XanaduAI/MrKite#196

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.55%. Comparing base (4acf7bd) to head (e72a84f). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #482 +/- ## ======================================== Coverage 89.55% 89.55% ======================================== Files 101 101 Lines 7384 7384 ======================================== Hits 6613 6613 Misses 771 771 ``` ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/482?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/482?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). Last update [4acf7bd...e72a84f](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/482?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI).