ENH: make the editable install build directory more discoverable

Change the editable install build directory to a subdirectory named
after the ABI tag for the target Python interpreter placed in the
``build`` directory in the project source directory.

Add .gitignode and .hgignore files with catch all glob patterns to the
``build`` directory, if it is empty, to have the major VCS systems
ignore it.

Fixes #370, #380.

mesonpy/__init__.py

@@ -1027,6 +1027,18 @@ def _check_meson_version(*, version: str = _MESON_REQUIRED_VERSION) -> None:
         raise ConfigError(f'Could not find meson version {version} or newer, found {meson_version}.')
 
 
+def _add_ignore_files(directory: pathlib.Path) -> None:
+    directory.joinpath('.gitignore').write_text(textwrap.dedent('''
+        # This file is generated by meson-python. It will not be recreated if deleted or modified.
+        *
+    '''), encoding='utf-8')
+    directory.joinpath('.hgignore').write_text(textwrap.dedent('''
+        # This file is generated by meson-python. It will not be recreated if deleted or modified.
+        syntax: glob
+        **/*
+    '''), encoding='utf-8')
+
+
 def _pyproject_hook(func: Callable[P, T]) -> Callable[P, T]:
     @functools.wraps(func)
     def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
@@ -1111,7 +1123,11 @@ def build_editable(
     if not config_settings:
         config_settings = {}
     if 'builddir' not in config_settings:
-        config_settings['builddir'] = os.path.join('.mesonpy', 'editable', 'build')
+        builddir = pathlib.Path('build')
+        builddir.mkdir(exist_ok=True)
+        if not next(builddir.iterdir(), None):
+            _add_ignore_files(builddir)
+        config_settings['builddir'] = os.fspath(builddir / str(mesonpy._tags.get_abi_tag()))
 
     out = pathlib.Path(wheel_directory)
     with _project(config_settings) as project: