Skip some tests if unpackaged files not present, make some files path-y-er

[mattwthompson]

The tests currently make some unsafe assumptions about files present relative to the Python package. For example, this does not necessarily point to the root of the repository, since many files are not packaged:

openff-toolkit/openff/toolkit/_tests/test_links.py

Lines 7 to 9 in 5ffcb58

	ROOT_DIR_PATH = os.path.join(
	os.path.dirname(os.path.realpath(__file__)), "..", "..", ".."
	)

From this, peeking at the README.md file (or looking for an examples/ directory) can introduce some surprises:

In [1]: import pathlib, os

In [2]: from openff.toolkit._tests.test_links import __file__

In [3]: os.system(f"head {pathlib.Path(__file__).parents[3] / 'README.md'}")
# autoflake

[![Build Status](https://github.com/PyCQA/autoflake/actions/workflows/main.yaml/badge.svg?branch=main)](https://github.com/PyCQA/autoflake/actions/workflows/main.yaml)

## Introduction

_autoflake_ removes unused imports and unused variables from Python code. It
makes use of [pyflakes](https://pypi.org/pypi/pyflakes) to do this.

By default, autoflake only removes unused imports for modules that are part of
Out[3]: 0

This change simply has fixtures return empty lists when it looks like tests are being collected from a packaged path (as distinct from a local path, where the full contents of the repository may be present).

[codecov]

Codecov Report

Merging #1825 (6c31044) into main (3c56d29) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

[j-wags]

Thanks @mattwthompson!

[j-wags]

Wow, trippy. I didn't realize that Path objects overrode the / operator. Made my day.