Build command supports multiple projects with one config

The create command already supports this.

src/towncrier/build.py

@@ -175,7 +175,9 @@ def __main(
     click.echo("Finding news fragments...", err=to_err)
 
     if config.directory is not None:
-        fragment_base_directory = os.path.abspath(config.directory)
+        fragment_base_directory = os.path.abspath(
+            os.path.join(base_directory, config.directory)
+        )
         fragment_directory = None
     else:
         fragment_base_directory = os.path.abspath(

src/towncrier/test/test_build.py

@@ -1344,3 +1344,37 @@ def test_with_topline_and_template_and_draft(self):
 
         self.assertEqual(0, result.exit_code, result.output)
         self.assertEqual(expected_output, result.output)
+
+    @with_isolated_runner
+    def test_multiple_projects_share_one_config(self, runner):
+        """
+        Multiple projects with independent changelogs share one towncrier
+        configuration.
+
+        For this to work:
+        1. We need to leave `config.package` empty.
+        2. We need to pass `--dir` to `create` and `build` explicitly.
+           It must point to the project folder.
+        3. We need to pass `--config` pointing at the global configuration.
+        4. We need to make sure `config.directory` and `config.filename` are resolved
+           relative to what we passed as `--dir`.
+        """
+        # We don't want to specify the package because we have multiple ones.
+        Path("pyproject.toml").write_text(
+            "[tool.towncrier]\n" + 'directory = "changelog.d"\n'
+        )
+        # Each subproject contains the source code...
+        Path("foo/foo").mkdir(parents=True)
+        Path("foo/foo/__init__.py").write_text("")
+        # ... and the changelog machinery.
+        Path("foo/changelog.d").mkdir()
+        Path("foo/changelog.d/123.feature").write_text("Adds levitation")
+        self.assertFalse(Path("foo/NEWS.rst").exists())
+
+        result = runner.invoke(
+            cli,
+            ("--yes", "--config", "pyproject.toml", "--dir", "foo", "--version", "1.0"),
+        )
+
+        self.assertEqual(0, result.exit_code)
+        self.assertTrue(Path("foo/NEWS.rst").exists())