Fix cache URLs on Windows

Closes #4479

The previous implementation would fail to install packages on Windows
because it creates a `Path` starting with a slash. Such a `Path` is
invalid on Windows. Instead, use the utility function url_to_path.

poetry/installation/executor.py

@@ -17,6 +17,7 @@
 from cleo.io.null_io import NullIO
 
 from poetry.core.packages.file_dependency import FileDependency
+from poetry.core.packages.utils.utils import url_to_path
 from poetry.core.packages.utils.link import Link
 from poetry.core.pyproject.toml import PyProjectTOML
 from poetry.utils._compat import decode