[#67] Move the declaration of pytest_plugins to a top-level conftest.py

This is what has been preventing us from running just the command "pytest"
instead of "pytest hlink/tests". pytest requires pytest_plugins to be
defined in a top-level conftest.py file. See here for their reasons:
https://docs.pytest.org/en/stable/deprecations.html#pytest-plugins-in-non-top-level-conftest-files.

conftest.py

@@ -0,0 +1,4 @@
+pytest_plugins = (
+    "hlink.tests.plugins.datasources",
+    "hlink.tests.plugins.external_data_paths",
+)

hlink/tests/conftest.py

@@ -23,12 +23,6 @@
 from types import SimpleNamespace
 
 
-pytest_plugins = (
-    "hlink.tests.plugins.datasources",
-    "hlink.tests.plugins.external_data_paths",
-)
-
-
 def load_table_from_csv(link_task, path, table_name):
     link_task.spark.read.csv(path, header=True, inferSchema=True).write.mode(
         "overwrite"