Support modern editable installs

[graingert-coef]

pip is switching to pyproject based editable installs pypa/pip#11457 which breaks the celery tests see #2735 (comment)

builds on top of #2735

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • move dash source code to src/dash
  • configure the project with new src layout
• I have run the tests locally and they passed. (refer to testing section in contributing)
• 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?