virtualenv: determine 'site-packages' based on implementation name

I'm not at all sure whether this is a correct fix or not, but it does
seem to make `pypy` work in at least some cases with `uv`. Previously,
I couldn't get it to work at all. Namely the virtualenv was created
with a `lib/python3.10/site-packages`, but whenever I did a `uv
pip install` in that virtualenv, it was looking for a non-existent
`lib/pypy3.10/site-packages` directory.

With this PR, the workflow reported as not working in #1488 now works
for me:

```
$ pypy3 --version
Python 3.10.13 (fc59e61cfbff, Jan 17 2024, 05:35:45)
[PyPy 7.3.15 with GCC 13.2.1 20230801]

$ uv venv --python $(which pypy3) --seed
Using Python 3.10.13 interpreter at: /usr/bin/pypy3
Creating virtualenv at: .venv
 + pip==24.0
 + setuptools==69.1.1
 + wheel==0.42.0
Activate with: source .venv/bin/activate

$ uv pip install 'alembic==1.0.11'
Resolved 9 packages in 8ms
Installed 9 packages in 14ms
 + alembic==1.0.11
 + greenlet==3.0.3
 + mako==1.3.2
 + markupsafe==2.1.5
 + python-dateutil==2.8.2
 + python-editor==1.0.4
 + six==1.16.0
 + sqlalchemy==2.0.27
 + typing-extensions==4.10.0
```

Where as previously (current `main`), I was hitting this error:

```
$ uv venv --python $(which pypy3) --seed
Using Python 3.10.13 interpreter at: /usr/bin/pypy3
Creating virtualenv at: .venv
 + pip==24.0
 + setuptools==69.1.1
 + wheel==0.42.0
Activate with: source .venv/bin/activate

$ uv pip install 'alembic==1.0.11'
error: Failed to list installed packages
  Caused by: failed to read directory `/home/andrew/astral/issues/uv/i1488/.venv/lib/pypy3.10/site-packages`
  Caused by: No such file or directory (os error 2)
```

Notice though that neither outcome above matches the error reported in #1488,
so this is likely not a complete fix. There are perhaps other lurking
issues.

Ref #1488

crates/uv-interpreter/src/interpreter.rs

@@ -93,9 +93,21 @@ impl Interpreter {
             // structure, which allows us to avoid querying the interpreter for the `sysconfig`
             // paths.
             sysconfig_paths: SysconfigPaths {
-                purelib: layout.site_packages(&venv_root, self.python_tuple()),
-                platlib: layout.site_packages(&venv_root, self.python_tuple()),
-                platstdlib: layout.platstdlib(&venv_root, self.python_tuple()),
+                purelib: layout.site_packages(
+                    &venv_root,
+                    self.site_packages_python(),
+                    self.python_tuple(),
+                ),
+                platlib: layout.site_packages(
+                    &venv_root,
+                    self.site_packages_python(),
+                    self.python_tuple(),
+                ),
+                platstdlib: layout.platstdlib(
+                    &venv_root,
+                    self.site_packages_python(),
+                    self.python_tuple(),
+                ),
                 scripts: layout.scripts(&venv_root),
                 data: layout.data(&venv_root),
                 ..self.sysconfig_paths
@@ -399,6 +411,33 @@ impl Interpreter {
         &self.sysconfig_paths.stdlib
     }
 
+    /// Return the name of the Python directory used to build the path to the
+    /// `site-packages` directory.
+    ///
+    /// If one could not be determined, then `python` is returned.
+    pub fn site_packages_python(&self) -> &str {
+        if self.implementation_name() == "pypy" {
+            "pypy"
+        } else {
+            "python"
+        }
+
+        // This implementation sniffs out what the directory name
+        // might be from sysconfig, but at the time of writing, this
+        // seems a little more risky than the simpler but less robust
+        // implementation above.
+        /*
+        const FALLBACK: &str = "python";
+        let Some(base) = self.include().file_name().and_then(|name| name.to_str()) else {
+            return FALLBACK;
+        };
+        base.char_indices()
+            .take_while(|(_, ch)| ch.is_ascii_alphabetic())
+            .last()
+            .map_or(FALLBACK, |(end, ch)| &base[..end + ch.len_utf8()])
+        */
+    }
+
     /// Return the [`Layout`] environment used to install wheels into this interpreter.
     pub fn layout(&self) -> Layout {
         Layout {
@@ -412,7 +451,8 @@ impl Interpreter {
                 // If the interpreter is a venv, then the `include` directory has a different structure.
                 // See: https://github.com/pypa/pip/blob/0ad4c94be74cc24874c6feb5bb3c2152c398a18e/src/pip/_internal/locations/_sysconfig.py#L172
                 self.prefix.join("include").join("site").join(format!(
-                    "python{}.{}",
+                    "{}{}.{}",
+                    self.site_packages_python(),
                     self.python_major(),
                     self.python_minor()
                 ))