feat(venv): add relocatable flag (#5515)

## Summary

Adds a `--relocatable` CLI arg to `uv venv`. This flag does two things:

* ensures that the associated activation scripts do not rely on a
hardcoded
absolute path to the virtual environment (to the extent possible; `.csh`
and
  `.nu` left as-is)
* persists a `relocatable` flag in `pyvenv.cfg`.

The flag in `pyvenv.cfg` in turn instructs the wheel `Installer` to
create script
entrypoints in a relocatable way (use `exec` trick + `dirname $0` on
POSIX;
use relative path to `python[w].exe` on Windows).

Fixes: #3863

## Test Plan

* Relocatable console scripts covered as additional scenarios in
existing test cases.
* Integration testing of boilerplate generation in `venv`.
* Manual testing of `uv venv` with and without `--relocatable`

crates/uv-python/src/environment.rs

@@ -162,6 +162,11 @@ impl PythonEnvironment {
         Ok(PyVenvConfiguration::parse(self.0.root.join("pyvenv.cfg"))?)
     }
 
+    /// Returns `true` if the environment is "relocatable".
+    pub fn relocatable(&self) -> bool {
+        self.cfg().is_ok_and(|cfg| cfg.is_relocatable())
+    }
+
     /// Returns the location of the Python executable.
     pub fn python_executable(&self) -> &Path {
         self.0.interpreter.sys_executable()

crates/uv-python/src/virtualenv.rs

@@ -28,6 +28,8 @@ pub struct PyVenvConfiguration {
     pub(crate) virtualenv: bool,
     /// If the uv package was used to create the virtual environment.
     pub(crate) uv: bool,
+    /// Is the virtual environment relocatable?
+    pub(crate) relocatable: bool,
 }
 
 #[derive(Debug, Error)]
@@ -136,6 +138,7 @@ impl PyVenvConfiguration {
     pub fn parse(cfg: impl AsRef<Path>) -> Result<Self, Error> {
         let mut virtualenv = false;
         let mut uv = false;
+        let mut relocatable = false;
 
         // Per https://snarky.ca/how-virtual-environments-work/, the `pyvenv.cfg` file is not a
         // valid INI file, and is instead expected to be parsed by partitioning each line on the
@@ -143,7 +146,7 @@ impl PyVenvConfiguration {
         let content = fs::read_to_string(&cfg)
             .map_err(|err| Error::ParsePyVenvCfg(cfg.as_ref().to_path_buf(), err))?;
         for line in content.lines() {
-            let Some((key, _value)) = line.split_once('=') else {
+            let Some((key, value)) = line.split_once('=') else {
                 continue;
             };
             match key.trim() {
@@ -153,11 +156,18 @@ impl PyVenvConfiguration {
                 "uv" => {
                     uv = true;
                 }
+                "relocatable" => {
+                    relocatable = value.trim().to_lowercase() == "true";
+                }
                 _ => {}
             }
         }
 
-        Ok(Self { virtualenv, uv })
+        Ok(Self {
+            virtualenv,
+            uv,
+            relocatable,
+        })
     }
 
     /// Returns true if the virtual environment was created with the `virtualenv` package.
@@ -169,4 +179,9 @@ impl PyVenvConfiguration {
     pub fn is_uv(&self) -> bool {
         self.uv
     }
+
+    /// Returns true if the virtual environment is relocatable.
+    pub fn is_relocatable(&self) -> bool {
+        self.relocatable
+    }
 }