installer: remove unrequested extras if requires_synchronization is set

[radoering]

tl;dr

• fix an issue where poetry install --sync does not remove unrequested extras (funny enough poetry install without --sync does remove them)
• increase test coverage of the installer regarding extras

• Added tests for changed code.
• Updated documentation for changed code.

Long story

Originally, I was just poking around with python-poetry/poetry-core#613 (comment). However, I noticed that all installer tests still passed if one of the following lines were removed:

• poetry/src/poetry/installation/installer.py

  Lines 386 to 388 in f5b0228

  	if locked.optional and locked.name not in extra_packages:
  	# Installed but optional and not requested in extras
  	ops.append(Uninstall(locked))

• poetry/src/poetry/installation/installer.py

  Lines 394 to 395 in f5b0228

  	if locked.optional and locked.name not in extra_packages:
  	continue

Thus, I decided to improve the test coverage before trying to fix bugs. I merged some quite similar tests and used pytest.mark.parametrize to cover the missing cases. However,

poetry/src/poetry/installation/installer.py

Lines 386 to 388 in f5b0228

	if locked.optional and locked.name not in extra_packages:
	# Installed but optional and not requested in extras
	ops.append(Uninstall(locked))

was still irrelevant.

After some code spelunking, I suppose that these lines became irrelevant with ea8fb8c. However, that introduced the mentioned bug. After adding @pytest.mark.parametrize("do_sync", [False, True]) to the test and fixing the bug, the mentioned lines of code are relevant again.

[Secrus]

LGTM

[github-actions]

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