Allow non-file:// paths to serve as --index-url values (#4524)

## Summary

pip allows these with the following logic:

```python
if os.path.exists(location):  # Is a local path.
    url = path_to_url(location)
    path = location
elif location.startswith("file:"):  # A file: URL.
    url = location
    path = url_to_path(location)
elif is_url(location):
    url = location
```

Closes https://github.com/astral-sh/uv/issues/4510.

## Test Plan

`cargo run pip install --index-url ../packse/index/simple-html/
example-a-961b4c22 --reinstall --no-cache --no-deps`

crates/distribution-types/src/index_url.rs

@@ -1,14 +1,15 @@
 use std::borrow::Cow;
 use std::fmt::{Display, Formatter};
 use std::ops::Deref;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::str::FromStr;
 
 use itertools::Either;
 use once_cell::sync::Lazy;
-use url::Url;
+use thiserror::Error;
+use url::{ParseError, Url};
 
-use pep508_rs::{expand_env_vars, split_scheme, strip_host, Scheme, VerbatimUrl};
+use pep508_rs::{expand_env_vars, split_scheme, strip_host, Scheme, VerbatimUrl, VerbatimUrlError};
 use uv_fs::normalize_url_path;
 
 use crate::Verbatim;
@@ -90,18 +91,40 @@ impl Verbatim for IndexUrl {
     }
 }
 
+/// An error that can occur when parsing an [`IndexUrl`].
+#[derive(Error, Debug)]
+pub enum IndexUrlError {
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+    #[error(transparent)]
+    Url(#[from] ParseError),
+    #[error(transparent)]
+    VerbatimUrl(#[from] VerbatimUrlError),
+    #[error("Index URL must be a valid base URL")]
+    CannotBeABase,
+}
+
 impl FromStr for IndexUrl {
-    type Err = url::ParseError;
+    type Err = IndexUrlError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let url = Url::parse(s)?;
+        if let Ok(path) = Path::new(s).canonicalize() {
-        let url = VerbatimUrl::from_url(url).with_given(s.to_owned());
+            let url = VerbatimUrl::from_path(path)?.with_given(s.to_owned());
-        if *url.raw() == *PYPI_URL {
-            Ok(Self::Pypi(url))
-        } else if url.scheme() == "file" {
             Ok(Self::Path(url))
         } else {
-            Ok(Self::Url(url))
+            let url = Url::parse(s)?;
+            if url.cannot_be_a_base() {
+                Err(IndexUrlError::CannotBeABase)
+            } else {
+                let url = VerbatimUrl::from_url(url).with_given(s.to_owned());
+                if *url.raw() == *PYPI_URL {
+                    Ok(Self::Pypi(url))
+                } else if url.scheme() == "file" {
+                    Ok(Self::Path(url))
+                } else {
+                    Ok(Self::Url(url))
+                }
+            }
         }
     }
 }