fix: fix support for packaging v20.4

Poetry v1.2.2 does not work with packaging v20.4
(which is an allowed version of packaging in the pyproject.toml file).

This is because the `from packaging.utils import NormalizedName` line
before [packaging v20.5][1]
only works if `typing.TYPE_CHECKING` is True.

This is not an issue on the `master` branch, since there this
import is already hidden behind an `if TYPE_CHECKING:` guard.
However, the v1.2 branch never cherry-picked the appropriate
commit adding this change.

[1]: pypa/packaging@eca30e0

Fixes: 04c5cbf

src/poetry/installation/installer.py

@@ -3,7 +3,6 @@
 from typing import TYPE_CHECKING
 
 from cleo.io.null_io import NullIO
-from packaging.utils import NormalizedName
 from packaging.utils import canonicalize_name
 
 from poetry.installation.executor import Executor
@@ -24,6 +23,7 @@
     from collections.abc import Sequence
 
     from cleo.io.io import IO
+    from packaging.utils import NormalizedName
     from poetry.core.packages.project_package import ProjectPackage
 
     from poetry.config.config import Config