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Install import hook earlier during pytest initialization (#143)

The pytest plugin does not work at all in this very common scenario:

- Application/library uses a conftest.py file with pytest configuration
- This conftest.py imports application/library modules.

Even worse, no matter what is passed as the ‘--typeguard-packages=’
command line argument, no warnings or errors will be reported at all.
Typeguard will be fully silent. this this makes it seem the code is
fully correct, while in reality the code is not checked by typeguard
*at all*. Oops. 💥 🙈 😱

The problem is that the pytest plugin used the ‘pytest_sessionstart()’
hook, which triggers only after conftest.py files (or other pytest
configuration files) have been loaded already. This is too late to
install the import hook: the targeted modules may have been imported
already at this point!

This pull request fixes this problem in the following way:

- Use ‘pytest_configure()’ instead of ‘pytest_sessionstart()’, since
  the former triggers before any conftest files are loaded.

- Make it impossible to silently skip checks for packages specified on
  the command line that cannot actually be checked. In those situations,
  raise an error with all the details. Example for
  ‘--typeguard-packages=sys’:

  INTERNALERROR> RuntimeError: typeguard cannot check these packages that are already imported: sys

See also:

- https://docs.pytest.org/en/stable/reference.html#pytest.hookspec.pytest_configure
- https://docs.pytest.org/en/stable/reference.html#pytest.hookspec.pytest_sessionstart

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