Unify editable and unnamed URL parsing (#3946)

## Summary

This will help prevent bugs like #3934 by unifying the implementations
for editables and non-editable unnamed requirements. Specifically, both
of these now go through the same parsing paths and use the same struct
representations (with the exception that the editable flag is flipped in
the first case):

```
-e ./foo/bar
./foo/bar
```

We also now support PEP 508 in editable URLs. It turns out this is just
a limitation in pip, so it's correct to support it. For example, this
now works:

```
-e black[d] @ file://${PROJECT_ROOT}/scripts/packages/black_editable
```

Closes #3941.

Closes #3942.

crates/pep508-rs/src/unnamed.rs

@@ -214,6 +214,12 @@ fn parse_unnamed_requirement<Url: UnnamedRequirementUrl>(
 
 /// Create a `VerbatimUrl` to represent the requirement, and extracts any extras at the end of the
 /// URL, to comply with the non-PEP 508 extensions.
+///
+/// For example:
+/// - `file:///home/ferris/project/scripts/...`
+/// - `file:../editable/`
+/// - `../editable/`
+/// - `https://download.pytorch.org/whl/torch_stable.html`
 fn preprocess_unnamed_url<Url: UnnamedRequirementUrl>(
     url: &str,
     #[cfg_attr(not(feature = "non-pep508-extensions"), allow(unused))] working_dir: Option<&Path>,
@@ -356,8 +362,39 @@ fn parse_unnamed_url<Url: UnnamedRequirementUrl>(
 ) -> Result<(Url, Vec<ExtraName>), Pep508Error<Url>> {
     // wsp*
     cursor.eat_whitespace();
+
     // <URI_reference>
-    let (start, len) = cursor.take_while(|char| !char.is_whitespace());
+    let (start, len) = {
+        let start = cursor.pos();
+        let mut len = 0;
+        let mut backslash = false;
+        let mut depth = 0u32;
+        while let Some((_, c)) = cursor.next() {
+            if backslash {
+                backslash = false;
+            } else if c == '\\' {
+                backslash = true;
+            } else if c == '[' {
+                depth = depth.saturating_add(1);
+            } else if c == ']' {
+                depth = depth.saturating_sub(1);
+            }
+
+            // If we see top-level whitespace, we're done.
+            if depth == 0 && c.is_whitespace() {
+                break;
+            }
+
+            // If we see a line break, we're done.
+            if matches!(c, '\r' | '\n') {
+                break;
+            }
+
+            len += c.len_utf8();
+        }
+        (start, len)
+    };
+
     let url = cursor.slice(start, len);
     if url.is_empty() {
         return Err(Pep508Error {

crates/requirements-txt/Cargo.toml

@@ -26,6 +26,7 @@ fs-err = { workspace = true }
 regex = { workspace = true }
 reqwest = { workspace = true, optional = true }
 reqwest-middleware = { workspace = true, optional = true }
+thiserror = { workspace = true }
 tracing = { workspace = true }
 unscanny = { workspace = true }
 url = { workspace = true }