Rebase: Uninstall existing non-editable versions when installing editable requirements bug (#682)

Separate branch for rebasing #677 onto main because i don't trust the
rebase enough to force push.

Closes #677.

---

If you install `black` from PyPI, then `-e ../black`, we need to
uninstall the existing `black`. This sounds simple, but that in turn
requires that we _know_ `-e ../black` maps to the package `black`, so
that we can mark it for uninstallation in the install plan. This, in
turn, means that we need to build editable dependencies prior to the
install plan.

This is just a bunch of reorganization to fix that specific bug
(installing multiple versions of `black` if you run through the above
workflow): we now run through the list of editables upfront, mark those
that are already installed, build those that aren't, and then ensure
that `InstallPlan` correctly removes those that need to be removed, etc.

Closes #676.

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

scripts/editable-installs/black_editable/black/__init__.py

@@ -0,0 +1,2 @@
+def a():
+    pass

scripts/editable-installs/black_editable/pyproject.toml

@@ -0,0 +1,12 @@
+[tool.poetry]
+name = "black"
+version = "0.1.0"
+description = ""
+authors = ["konstin <konstin@mailbox.org>"]
+
+[tool.poetry.dependencies]
+python = "^3.10"
+
+[build-system]
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"