Move per-module tests under each module

Summary:
Reorganize SPDL tests so each module owns its own tests. Tests previously lived under `fbcode/spdl/tests/<module>/` and depended on a shared root for fixtures and Buck macros. After this change, each module's tests live in `<module>/tests/`, which makes `buck test fbcode//spdl/<module>/...` run every test for that module without enumerating sibling test paths.

New layout:
- `spdl/io/tests/` (was `spdl/tests/io/`) — also hosts the shared `fixture.py` + `fb/__init__.py` (FFmpeg helpers), since only `io` and `cuda` tests use them and `cuda` tests already depend only on `spdl.io`.
- `spdl/cuda/tests/` (was `spdl/tests/cuda/`) — `PACKAGE` moved with it; depends on `//spdl/io/tests:fixture` and imports `from spdl.io.tests.fixture`.
- `spdl/pipeline/tests/` (was `spdl/tests/pipeline/`, including `fb/`).
- `spdl/dataloader/tests/` (was `spdl/tests/dataloader/`).
- `spdl/autoresearch/tests/` (was `spdl/tests/autoresearch/`, including `fb/`).
- `spdl/def.bzl` — new file holding the `spdl_tests_with_ffmpeg_variants` and `spdl_tests_with_python_variants` macros that were previously in `spdl/tests/def.bzl`.
- `fbcode/spdl/tests/` deleted entirely.

Other updates:
- `from ..fixture` (relative) is now `from .fixture` for `io` tests and `from spdl.io.tests.fixture` (absolute) for `cuda` tests.
- `spdl.io.tests/fb/__init__.py` resolves the FFmpeg resource via `importlib.resources.path("spdl.io.tests", "ffmpeg")`.
- Visibility lists in module BUCKs (`spdl/io/...`, `spdl/io/lib/...`, `spdl/io/utils/...`, `spdl/pipeline/...`, `spdl/pipeline/_iter_utils/...`, `spdl/pipeline/fb/...`, `spdl/pipeline/fb/lib/...`, `spdl/autoresearch/...`, `spdl/autoresearch/_app/...`, `spdl/autoresearch/_common/fb/...`, `spdl/autoresearch/core/...`, `spdl/autoresearch/pipeline_optimization/...`) updated to point to the new per-module test paths.
- `fbcode/spdl/PACKAGE` comments updated to reference the new `tests/` paths.
- `fbcode/spdl/.llms/skills/code-authoring/SKILL.md` updated to describe the new layout convention.
- GitHub workflows (`_build_linux.yml`, `_build_linux_cuda.yml`, `_build_macos.yml`, `_build_windows.yml`) updated so each pytest invocation lists the new per-module paths under the OSS `src/spdl/` prefix; `docs.yml` `paths-ignore` updated to `src/spdl/**/tests/**`.
- Hydra `_target_` string in `pipeline_hydra_test.py` updated from `spdl.tests.pipeline.fb...` to `spdl.pipeline.tests.fb...`.

Differential Revision: D107140706

Author: mthrok

.github/workflows/_build_linux.yml

@@ -177,10 +177,10 @@ jobs:
 
           python -c 'import spdl.io.utils;assert not spdl.io.utils.built_with_cuda()'
           pytest -v -n ${{ inputs.test-concurrency }} \
-              tests/io/ \
-              tests/dataloader/ \
-              tests/pipeline/ \
-              tests/autoresearch/
+              src/spdl/io/tests/ \
+              src/spdl/dataloader/tests/ \
+              src/spdl/pipeline/tests/ \
+              src/spdl/autoresearch/tests/
 
   stub-check:
     needs: ["build"]

.github/workflows/_build_linux_cuda.yml

@@ -221,8 +221,8 @@ jobs:
               python -c 'import spdl.io.utils;assert spdl.io.utils.built_with_nvcodec()'
           fi
           pytest -v -n ${{ inputs.test-concurrency }} \
-              tests/cuda/ \
-              tests/io/
+              src/spdl/cuda/tests/ \
+              src/spdl/io/tests/
 
   test-cpu:
     if: "${{ inputs.run-test == 'true' }}"
@@ -284,10 +284,10 @@ jobs:
           set -ex
 
           pytest -v -n ${{ inputs.test-concurrency }} \
-              tests/io \
-              tests/dataloader \
-              tests/pipeline \
-              tests/autoresearch
+              src/spdl/io/tests \
+              src/spdl/dataloader/tests \
+              src/spdl/pipeline/tests \
+              src/spdl/autoresearch/tests
 
   stub-check:
     needs: ["build"]

.github/workflows/_build_macos.yml

@@ -128,7 +128,7 @@ jobs:
         run: |
           set -ex
           pytest -v -n ${{ inputs.test-concurrency }} \
-              tests/io \
-              tests/dataloader \
-              tests/pipeline \
-              tests/autoresearch
+              src/spdl/io/tests \
+              src/spdl/dataloader/tests \
+              src/spdl/pipeline/tests \
+              src/spdl/autoresearch/tests

.github/workflows/_build_windows.yml

@@ -134,7 +134,7 @@ jobs:
         run: |
           set -ex
           pytest -v -n ${{ inputs.test-concurrency }} \
-              tests/io \
-              tests/dataloader \
-              tests/pipeline \
-              tests/autoresearch
+              src/spdl/io/tests \
+              src/spdl/dataloader/tests \
+              src/spdl/pipeline/tests \
+              src/spdl/autoresearch/tests

.github/workflows/docs.yml

@@ -4,14 +4,14 @@ on:
   workflow_dispatch:
   pull_request:
     paths-ignore:
-      - "tests/**"
+      - "src/spdl/**/tests/**"
       - "third_party/**"
       - "*.md"
     branches:
       - main
   push:
     paths-ignore:
-      - "tests/**"
+      - "src/spdl/**/tests/**"
       - "third_party/**"
       - "*.md"
     branches: