Add support for the file:// URL scheme via requests_file's FileAdapter,
so the index/package URL can point at a local on-disk PyPI mirror
instead of only a remote HTTP(S) index.

requests_file >= 2.1.0 is required: earlier releases don't set
Response.request in FileAdapter.send(), so _cache.py's use of
response.request.url raises AttributeError: 'NoneType' object has
no attribute 'url'.

Obtained from: https://github.com/michael-o/proxpi/commit/14ea92360f051723f415e9c2d622d07a18946ff1

--- pyproject.toml.orig
+++ pyproject.toml
@@ -25,6 +25,7 @@ dependencies = [
     "jinja2 ~= 3.0",
     "lxml >= 4.8, < 7.0",
     "requests ~= 2.27",
+    "requests_file >= 2.1.0",
 ]
 description = "PyPI caching mirror"
 keywords = ["pypi", "index", "mirror", "cache"]
--- src/proxpi/_cache.py.orig
+++ src/proxpi/_cache.py
@@ -16,6 +16,7 @@
 import urllib.parse

 import requests
+from requests_file import FileAdapter
 import lxml.etree

 INDEX_URL = os.environ.get("PROXPI_INDEX_URL", "https://pypi.org/simple/")
@@ -484,6 +485,7 @@ def __init__(self, index_url: str, ttl: int, session: requests.Session = None):
         self.index_url = index_url
         self.ttl = ttl
         self.session = session or requests.Session()
+        self.session.mount("file://", FileAdapter())
         self._index_t = None
         self._index_lock = threading.Lock()
         self._package_locks = _Locks()
@@ -503,7 +505,11 @@ def _list_packages(self):
         self._stats.add_miss(key="<index>")

         logger.info(f"Listing packages in index '{self._index_url_masked}'")
-        response = self.session.get(self.index_url, headers=self._headers, stream=True)
+        index_url = self.index_url
+        splitted_index_url = urllib.parse.urlsplit(self.index_url)
+        if splitted_index_url.scheme == "file" and os.path.isdir(urllib.parse.unquote(splitted_index_url.path)):
+            index_url = urllib.parse.urljoin(self.index_url, "index.html")
+        response = self.session.get(index_url, headers=self._headers, stream=True)
         response.raise_for_status()
         self._index_t = _now()

@@ -576,6 +582,9 @@ def _list_files(self, package_name: str):
                 raise NotFound(package_name)
             package_url = self._index[package_name]
             url = urllib.parse.urljoin(self.index_url, package_url)
+            splitted_url = urllib.parse.urlsplit(url)
+            if splitted_url.scheme == "file" and os.path.isdir(urllib.parse.unquote(splitted_url.path)):
+                url = urllib.parse.urljoin(url, "index.html")
             response = self.session.get(url, headers=self._headers, stream=True)
             response.raise_for_status()

@@ -757,6 +766,7 @@ def __init__(
         self.cache_dir = os.path.abspath(cache_dir or tempfile.mkdtemp())
         self.download_timeout = download_timeout
         self.session = session or requests.Session()
+        self.session.mount("file://", FileAdapter())
         self._cache_dir_provided = cache_dir
         self._files = {}
         self._evict_lock = threading.Lock()
@@ -794,7 +804,7 @@ def _populate_files_from_existing_cache_dir(self):
     def _get_key(url: str) -> str:
         """Get file cache reference key from file URL."""
         urlsplit = urllib.parse.urlsplit(url)
-        parent = _hostname_normalise_pattern.sub("-", urlsplit.hostname)
+        parent = _hostname_normalise_pattern.sub("-", urlsplit.hostname) if urlsplit.hostname else "local"
         return posixpath.join(parent, *_split_path(urlsplit.path, posixpath.split))

     def _download_file(self, url: str, path: str):