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-virtualenv/src/virtualenv.rs

@@ -50,6 +50,7 @@ pub(crate) fn create(
     prompt: Prompt,
     system_site_packages: bool,
     allow_existing: bool,
+    relocatable: bool,
 ) -> Result<VirtualEnvironment, Error> {
     // Determine the base Python executable; that is, the Python executable that should be
     // considered the "base" for the virtual environment. This is typically the Python executable
@@ -294,12 +295,27 @@ pub(crate) fn create(
         .map(|path| path.simplified().to_str().unwrap().replace('\\', "\\\\"))
         .join(path_sep);
 
-        let activator = template
-            .replace(
-                "{{ VIRTUAL_ENV_DIR }}",
+        let virtual_env_dir = match (relocatable, name.to_owned()) {
+            (true, "activate") => {
+                // Extremely verbose, but should cover all major POSIX shells,
+                // as well as platforms where `readlink` does not implement `-f`.
+                r#"'"$(dirname -- "$(CDPATH= cd -- "$(dirname -- ${BASH_SOURCE[0]:-${(%):-%x}})" && echo "$PWD")")"'"#
+            }
+            (true, "activate.bat") => r"%~dp0..",
+            (true, "activate.fish") => {
+                r#"'"$(dirname -- "$(cd "$(dirname -- "$(status -f)")"; and pwd)")"'"#
+            }
+            // Note:
+            // * relocatable activate scripts appear not to be possible in csh and nu shell
+            // * `activate.ps1` is already relocatable by default.
+            _ => {
                 // SAFETY: `unwrap` is guaranteed to succeed because `location` is an `Utf8PathBuf`.
-                location.simplified().to_str().unwrap(),
-            )
+                location.simplified().to_str().unwrap()
+            }
+        };
+
+        let activator = template
+            .replace("{{ VIRTUAL_ENV_DIR }}", virtual_env_dir)
             .replace("{{ BIN_NAME }}", bin_name)
             .replace(
                 "{{ VIRTUAL_PROMPT }}",
@@ -335,6 +351,14 @@ pub(crate) fn create(
                 "false".to_string()
             },
         ),
+        (
+            "relocatable".to_string(),
+            if relocatable {
+                "true".to_string()
+            } else {
+                "false".to_string()
+            },
+        ),
     ];
 
     if let Some(prompt) = prompt {