feat: Add PyPI as the default index in PackageFinder

Signed-off-by: Frost Ming <me@frostming.com>

src/unearth/__main__.py

@@ -161,7 +161,7 @@ def cli(argv: list[str] | None = None) -> None:
     name = args.requirement.name
     target_python = TargetPython(args.py_ver, args.abis, args.impl, args.platforms)
     finder = PackageFinder(
-        index_urls=args.index_urls or ["https://pypi.org/simple/"],
+        index_urls=args.index_urls or [],
         find_links=args.find_links or [],
         trusted_hosts=args.trusted_hosts or [],
         target_python=target_python,

src/unearth/finder.py

@@ -95,6 +95,9 @@ def __init__(
             self.add_index_url(url)
         for url in find_links:
             self.add_find_links(url)
+        # Add PyPI as the default index.
+        if not self.sources:
+            self.add_index_url("https://pypi.org/simple/")
         self.target_python = target_python or TargetPython()
         self.ignore_compatibility = ignore_compatibility
         self.no_binary = {canonicalize_name(name) for name in no_binary}