More plugins: splashscreen and importmap (#938)

This PR move codes from main.ts into two new plugins:

- splashscreen (formerly known as py-loader)
- importmap

The old setting config.autoclose_loader is still supported but deprecated; the new setting is config.splashscreen.autoclose.

Moreover, it does a small refactoring around UserError: now UserErrors are correctly caught even if they are raised from within afterRuntimeLoad.

pyscriptjs/tests/integration/test_py_config.py

@@ -5,7 +5,7 @@ import tempfile
 import pytest
 import requests
 
-from .support import JsErrors, PyScriptTest
+from .support import PyScriptTest
 
 URL = "https://github.com/pyodide/pyodide/releases/download/0.20.0/pyodide-build-0.20.0.tar.bz2"
 TAR_NAME = "pyodide-build-0.20.0.tar.bz2"
@@ -245,30 +245,20 @@ class TestConfig(PyScriptTest):
             wait_for_pyscript=False,
         )
 
-        # This is expected if running pytest with --dev flag
-        localErrorContent = """PyScript: Access to local files
+        if self.is_fake_server:
+            expected = """PyScript: Access to local files
         (using "Paths:" in <py-config>)
         is not available when directly opening a HTML file;
         you must use a webserver to serve the additional files."""
-
-        # This is expected if running a live server
-        serverErrorContent = (
-            "Loading from file <u>./f.py</u> failed with error 404 (File not Found). "
-            "Are your filename and path are correct?"
-        )
+        else:
+            expected = (
+                "Loading from file <u>./f.py</u> failed with error 404 (File not Found). "
+                "Are your filename and path are correct?"
+            )
 
         inner_html = self.page.locator(".py-error").inner_html()
-        assert localErrorContent in inner_html or serverErrorContent in inner_html
-        assert "Failed to load resource" in self.console.error.lines[0]
-        assert "Caught an error in fetchPaths" in self.console.warning.lines[0]
-        with pytest.raises(JsErrors) as exc:
-            self.check_js_errors()
-
-        received_error_msg = str(exc.value)
-        assert (
-            localErrorContent in received_error_msg
-            or serverErrorContent in received_error_msg
-        )
+        assert expected in inner_html
+        assert expected in self.console.error.lines[-1]
 
     def test_paths_from_packages(self):
         self.writefile("utils/__init__.py", "")