Add missing as_posix() calls

As PR python-poetry#2398 changed package.root_dir from str to Path, we need to call
as_posix() to it convert to str before passing to os.path.join.

poetry/installation/pip_installer.py

@@ -143,7 +143,7 @@ def requirement(self, package, formatted=False):
 
         if package.source_type in ["file", "directory"]:
             if package.root_dir:
-                req = os.path.join(package.root_dir, package.source_url)
+                req = os.path.join(package.root_dir.as_posix(), package.source_url)
             else:
                 req = os.path.realpath(package.source_url)
 
@@ -184,7 +184,7 @@ def install_directory(self, package):
         from poetry.utils.toml_file import TomlFile
 
         if package.root_dir:
-            req = os.path.join(package.root_dir, package.source_url)
+            req = os.path.join(package.root_dir.as_posix(), package.source_url)
         else:
             req = os.path.realpath(package.source_url)