Append test plugin packages via monkeypatch.syspath_prepend

[tony]

Fixes #658

• 1st commit : Applies monkey patch globally to all tests, removes test packages from pyproject.toml

• 2nd commit : Scopes it to only relevant packages, so unrelated tests don't have sys.path's altered.

  This is optional, but good to ensure explicitness (down the road, implicit alteration of site packages for all tests may be super hard to track down if it leads to an error in tests years from now, so I try to use autouse=True sparingly as possible)

Any thoughts on this approach @joseph-flinn?

[joseph-flinn]

@tony I like this approach much better than what I had. Testing is probably my weakest technical skill and monkeypatch is still a pretty big black box to me.

[tony]

@joseph-flinn Then let's give this a shot! I will backport and publish versions for 1.6 and 1.7 to see if that works for #658 (and any other maintainers that bumped into this issue)