optimize buildkite python packages change detection (#22141)

## Summary & Motivation

Determining buildkite pipeline steps locally is very slow (22s). I believe this is most likely due to __pycache__ directories or other similar files that are probably not an issue in CI. However, when iterating locally, it helps for this step to be as fast as possible, and these changes reduce local pipeline output from 22s to 2s. 

## How I Tested These Changes

- visually inspected that the same packages are found
- modified a python file in a package and ensured that it was still picked up as a changed package

.buildkite/dagster-buildkite/dagster_buildkite/python_packages.py

@@ -1,19 +1,25 @@
 # pyright: reportUnnecessaryTypeIgnoreComment=false
 
 import logging
+import os
+import subprocess
 from distutils import core as distutils_core
 from importlib import reload
 from pathlib import Path
 from typing import Dict, Optional, Set
 
-import pathspec
 from pkg_resources import Requirement, parse_requirements
 
 from dagster_buildkite.git import ChangedFiles, GitInfo
 
 changed_filetypes = [".py", ".cfg", ".toml", ".yaml", ".ipynb", ".yml", ".ini", ".jinja"]
 
 
+def _path_is_relative_to(p: Path, u: Path) -> bool:
+    # see https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.is_relative_to
+    return u == p or u in p.parents
+
+
 class PythonPackage:
     def __init__(self, setup_py_path: Path):
         self.directory = setup_py_path.parent
@@ -114,22 +120,16 @@ def load_from_git(cls, git_info: GitInfo) -> None:
 
         logging.info("Finding Python packages:")
 
-        git_ignore = git_info.directory / ".gitignore"
-
-        if git_ignore.exists():
-            ignored = git_ignore.read_text().splitlines()
-            git_ignore_spec = pathspec.PathSpec.from_lines("gitwildmatch", ignored)
-        else:
-            git_ignore_spec = pathspec.PathSpec([])
-
         # Consider any setup.py file to be a package
-        packages = set(
-            [
-                PythonPackage(Path(setup))
-                for setup in git_info.directory.rglob("setup.py")
-                if not git_ignore_spec.match_file(str(setup))
-            ]
-        )
+        output = subprocess.check_output(
+            ["git", "ls-files", "."],
+            cwd=str(git_info.directory),
+        ).decode("utf-8")
+        packages = [
+            PythonPackage(git_info.directory / Path(file))
+            for file in output.split("\n")
+            if os.path.basename(file) == "setup.py"
+        ]
 
         for package in sorted(packages):
             logging.info("  - " + package.name)
@@ -142,7 +142,7 @@ def load_from_git(cls, git_info: GitInfo) -> None:
             for change in ChangedFiles.all:
                 if (
                     # Our change is in this package's directory
-                    (change in package.directory.rglob("*"))
+                    _path_is_relative_to(change, package.directory)
                     # The file can alter behavior - exclude things like README changes
                     and (change.suffix in changed_filetypes)
                     # The file is not part of a test suite. We treat this differently