fix(python-uv): cached build copies dependencies to the build dir

[tomaDev]

Issue

Fixes #864.

`sam build --cached --beta-features` against a project using the
python-uv workflow fails with:

```
Build Failed
Error: PythonUvBuilder:CopyDependencies - [Errno 2] No such file or directory: '.aws-sam/build/'
```

The failure is also destructive — it wipes `.aws-sam/build/` for every
function, forcing a full `sam build --beta-features` to recover.

Root cause

`sam build --cached` runs the workflow with a `dependencies_dir` set
(`.aws-sam/deps/`) so deps can be reused across builds. Two bugs
on that path:

1. `PythonUvWorkflow._setup_build_actions` constructs `CopyDependenciesAction` with the source/destination swapped. Dependencies are installed into `dependencies_dir`, so that is the source of the copy and `artifacts_dir` the destination. As written, the action listed the not-yet-created `artifacts_dir` as source and never populated it — hence the `[Errno 2]`.

2. `UvRunner.install_requirements` passes `--target` to UV verbatim. UV runs with `cwd` set to the project directory, so a relative `.aws-sam/deps/` target is resolved under the source dir, not the build root. `uv pip install` succeeds but writes to the wrong place — the cached path then can't find the deps.

Fix

• Swap the `CopyDependenciesAction` arguments in `workflow.py` so it copies from `dependencies_dir` → `artifacts_dir`.
• Resolve `--target` to an absolute path before invoking UV in `packager.py`.

Tests

• Unit test in `test_workflow.py` asserts `CopyDependenciesAction` is constructed with the correct source/artifact/destination wiring.
• Unit test in `test_packager.py` asserts a relative `--target` is made absolute.
• Existing `dependencies_dir` integration test now asserts deps end up in `artifacts_dir` (previously only checked that the build didn't crash).

Verified locally: `sam build --cached --parallel --beta-features` now
succeeds end-to-end on a 9-function python-uv project.