chore: add a get_data_dir() function to core.dirs

This wraps `platformdirs.user_data_dir()`. It's going to be used as the
place to store our global git hook.

Changing the place of the global git hook is required to make the code
testable. It is also going to fix the fact that having the global git
hooks created in $HOME/.git/hooks was weird: for 99% of our users, $HOME is
not a git repository.

Author: agateau-gg

ggshield/core/dirs.py

@@ -1,7 +1,7 @@
 import os
 from pathlib import Path
 
-from platformdirs import user_cache_dir, user_config_dir
+from platformdirs import user_cache_dir, user_config_dir, user_data_dir
 
 from ggshield.utils.git_shell import NotAGitDirectory, get_git_root
 
@@ -34,6 +34,16 @@ def get_cache_dir() -> Path:
         return Path(user_cache_dir(appname=APPNAME, appauthor=APPAUTHOR))
 
 
+def get_data_dir() -> Path:
+    try:
+        # See tests/conftest.py for details
+        return Path(os.environ["GG_DATA_DIR"])
+    except KeyError:
+        return Path(
+            user_data_dir(appname=APPNAME, appauthor=APPAUTHOR)
+        )  # pragma: no cover
+
+
 def get_project_root_dir(path: Path) -> Path:
     """
     Returns the source basedir required to find file within filesystem.

tests/conftest.py

@@ -86,6 +86,7 @@ def do_not_use_real_user_dirs(monkeypatch, tmp_path):
     """
     monkeypatch.setenv("GG_CONFIG_DIR", str(tmp_path / "config"))
     monkeypatch.setenv("GG_CACHE_DIR", str(tmp_path / "cache"))
+    monkeypatch.setenv("GG_DATA_DIR", str(tmp_path / "data"))
     monkeypatch.setenv("GG_USER_HOME_DIR", str(tmp_path / "home"))