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-cli/src/lib.rs

@@ -1743,6 +1743,22 @@ pub struct VenvArgs {
     #[arg(long)]
     pub system_site_packages: bool,
 
+    /// Make the virtual environment relocatable.
+    ///
+    /// A relocatable virtual environment can be moved around and redistributed without
+    /// invalidating its associated entrypoint and activation scripts.
+    ///
+    /// Note that this can only be guaranteed for standard `console_scripts` and `gui_scripts`.
+    /// Other scripts may be adjusted if they ship with a generic `#!python[w]` shebang,
+    /// and binaries are left as-is.
+    ///
+    /// As a result of making the environment relocatable (by way of writing relative, rather than
+    /// absolute paths), the entrypoints and scripts themselves will _not_ be relocatable. In other
+    /// words, copying those entrypoints and scripts to a location outside the environment will not
+    /// work, as they reference paths relative to the environment itself.
+    #[arg(long)]
+    pub relocatable: bool,
+
     #[command(flatten)]
     pub index_args: IndexArgs,