Consolidate all package handling logic into hook module

This creates a large module to sort through, but the hope is that it
avoids the need to constantly hop around without rhyme or reason to
find the piece of logic you're looking for. The module structure is
mapped to functionality rather than an arbitrary concept of reducing
line number.

tests/fixtures.py

@@ -10,7 +10,7 @@ import pytest
 import tox.tox_env.python.virtual_env.runner
 from poetry.installation.operations.operation import Operation
 
-from tox_poetry_installer import utilities
+import tox_poetry_installer.hooks._tox_on_install_helpers
 
 
 TEST_PROJECT_PATH = Path(__file__).parent.resolve() / "test-project"
@@ -47,7 +47,11 @@ class MockExecutor:
 
 @pytest.fixture
 def mock_venv(monkeypatch):
-    monkeypatch.setattr(utilities, "convert_virtualenv", lambda venv: venv)
+    monkeypatch.setattr(
+        tox_poetry_installer.hooks._tox_on_install_helpers,
+        "convert_virtualenv",
+        lambda venv: venv,
+    )
     monkeypatch.setattr(poetry.installation.executor, "Executor", MockExecutor)
     monkeypatch.setattr(
         tox.tox_env.python.virtual_env.runner, "VirtualEnvRunner", MockVirtualEnv