pytest silently chooses the wrong fixture when two plugins declare a fixture with the same name

The magic name-matching of pytest's fixtures can lead to namespace clashes, apart from being unpythonic (see #3834). Getting a wrong fixture may lead to hard-to-debug errors, especially if one does not know the plugin code and therefore does not know if one plugin may legitimately call the other plugin's code. Thus, I believe that pytest should not allow a fixture to be used whose name clashes between two declarations (Python Zen: In the face of ambiguity, refuse the temptation to guess.). Instead, it should raise an explicit error, including a hint to the test-writer how they may resolve the ambiguity.

The environment where this bug was observed was the following:

python 3.6.1
This is pytest version 3.5.0
  pytest-sanic-0.1.13
  pytest-benchmark-3.1.1
  pytest-aiohttp-0.3.0

where both aiohttp and sanic provide a test_client fixture.

Start by reproducing the fixture collision with pytest-sanic, pytest-benchmark, and pytest-aiohttp, focusing on the competing test_client fixtures described in the issue. Trace how pytest selects fixtures and define the explicit error and resolution hint; done means ambiguous declarations no longer silently select one fixture.