Move install function to dedicated submodule

Fix duplicate package installs caused by using list for ordering

tox_poetry_installer/installer.py

@@ -0,0 +1,53 @@
+"""Funcationality for performing virtualenv installation"""
+# Silence this one globally to support the internal function imports for the proxied poetry module.
+# See the docstring in 'tox_poetry_installer._poetry' for more context.
+# pylint: disable=import-outside-toplevel
+import typing
+from pathlib import Path
+from typing import Sequence
+from typing import Set
+
+import tox
+from poetry.core.packages import Package as PoetryPackage
+from tox.venv import VirtualEnv as ToxVirtualEnv
+
+from tox_poetry_installer import constants
+
+if typing.TYPE_CHECKING:
+    from tox_poetry_installer import _poetry
+
+
+def install(
+    poetry: "_poetry.Poetry", venv: ToxVirtualEnv, packages: Sequence[PoetryPackage]
+):
+    """Install a bunch of packages to a virtualenv
+
+    :param poetry: Poetry object the packages were sourced from
+    :param venv: Tox virtual environment to install the packages to
+    :param packages: List of packages to install to the virtual environment
+    """
+    from tox_poetry_installer import _poetry
+
+    tox.reporter.verbosity1(
+        f"{constants.REPORTER_PREFIX} Installing {len(packages)} packages to environment at {venv.envconfig.envdir}"
+    )
+
+    pip = _poetry.PipInstaller(
+        env=_poetry.VirtualEnv(path=Path(venv.envconfig.envdir)),
+        io=_poetry.NullIO(),
+        pool=poetry.pool,
+    )
+
+    installed: Set[PoetryPackage] = set()
+
+    for dependency in packages:
+        if dependency not in installed:
+            tox.reporter.verbosity1(
+                f"{constants.REPORTER_PREFIX} Installing {dependency}"
+            )
+            pip.install(dependency)
+            installed.add(dependency)
+        else:
+            tox.reporter.verbosity1(
+                f"{constants.REPORTER_PREFIX} Already installed {dependency}, skipping"
+            )