Support modern editable installs

[graingert-coef]

pip is switching to pyproject based editable installs https://github.com/pypa/pip/issues/11457 which breaks the celery tests see https://github.com/plotly/dash/pull/2735#discussion_r1480010572

builds on top of https://github.com/plotly/dash/pull/2735

Contributor Checklist

• [x] I have broken down my PR scope into the following TODO tasks
  • [x] move dash source code to src/dash
  • [x] configure the project with new src layout
• [x] I have run the tests locally and they passed. (refer to testing section in contributing)
• [x] I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

I don't think this needs a changelog entry

optionals

• [ ] I have added entry in the CHANGELOG.md
• [ ] If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • [ ] this GitHub #PR number updates the dash docs
  • [ ] here is the show and tell thread in Plotly Dash community

[gvwilson]

@graingert-coef thank you again for this PR - we're rethinking our build environments right now, so we will revisit it in the next cycle.

[marthacryan]

Thank you for the PR! Could you undo the change that moves files into the src directory?

[robertclaus]

Closing due to age, inactivity, and the fact that this should be resolved in https://github.com/plotly/dash/pull/3336