Move uvx shell completion to uvx --generate-shell-completion (#7511)

## Summary

Because a problem was found with Powershell and combining the generated
completion scripts for uv and uvx, let's try separating uv and uvx
command completion scripts.

The generated powershell script template can be seen in clap_complete
source, and it starts with `using` directives, which makes it impossible
(apparently) to concatenate two of those script outputs.

As a side effect, this is available under `uv tool run
--generate-shell-completion` too.

Fixes #7482

## Test Plan

- `eval "$(cargo run --bin uvx -- --generate-shell-completion bash)"`
- Test Powershell

.github/workflows/ci.yml

@@ -197,6 +197,13 @@ jobs:
           $uv venv
           $uv pip install ruff
 
+      - name: "Smoke test completion"
+        run: |
+          uv="./target/debug/uv"
+          uvx="./target/debug/uvx"
+          eval "$($uv generate-shell-completion bash)"
+          eval "$($uvx --generate-shell-completion bash)"
+
   cargo-test-macos:
     timeout-minutes: 10
     needs: determine_changes
@@ -296,6 +303,18 @@ jobs:
           uv venv
           uv pip install ruff
 
+      - name: "Smoke test completion"
+        working-directory: ${{ env.UV_WORKSPACE }}
+        shell: powershell
+        env:
+          # Avoid debug build stack overflows.
+          UV_STACK_SIZE: 2000000
+        run: |
+          Set-Alias -Name uv -Value ./target/debug/uv
+          Set-Alias -Name uvx -Value ./target/debug/uvx
+          (& uv generate-shell-completion powershell) | Out-String | Invoke-Expression
+          (& uvx --generate-shell-completion powershell) | Out-String | Invoke-Expression
+
   # Separate jobs for the nightly crate
   windows-trampoline-check:
     timeout-minutes: 10