chore: Add test to verify shiny isn't importing itself internally

tests/pytest/test_shiny_import.py

@@ -0,0 +1,80 @@
+from __future__ import annotations
+
+import glob
+from pathlib import Path
+
+from tests.pytest._utils import skip_on_windows
+
+
+class BadLine:
+    def __init__(self, path: Path, line_number: int, line_text: str):
+        self.path = path
+        self.line_number = line_number
+        self.line_text = line_text
+
+    def __repr__(self):
+        return f"{self.path}:{self.line_number} - {self.line_text}"
+
+
+@skip_on_windows
+def test_shiny_import_itself():
+    """
+    VSCode likes to import from the shiny module itself within the shiny package. While it works... it relies on import magic, not relative imports.
+
+    Bad: `from shiny import ui`
+    Good: `from . import ui`
+    """
+
+    root_here = Path(__file__).parent.parent.parent
+    shiny_files = glob.glob(str(root_here / "shiny" / "**" / "*.py"), recursive=True)
+
+    shiny_files = [
+        path
+        for path in shiny_files
+        if "/api-examples/" not in path
+        and "/templates/" not in path
+        and Path(path).is_file()
+    ]
+
+    assert len(shiny_files) > 0
+
+    # bad_entries: list[tuple[Path, int, str]] = []
+    bad_entries: list[BadLine] = []
+
+    # For every python file...
+    for path in shiny_files:
+        path = Path(path)
+
+        file_txt = path.read_text(encoding="utf-8")
+        while True:
+            if "\ndef " in file_txt:
+                file_txt = file_txt.split("\ndef ")[0]
+            elif "\nasync def " in file_txt:
+                file_txt = file_txt.split("\nasync def ")[0]
+            elif "\nclass " in file_txt:
+                file_txt = file_txt.split("\nclass ")[0]
+            else:
+                break
+
+        for search_txt in ("\nfrom shiny.", "\nfrom shiny ", "\nimport shiny\n"):
+            if search_txt == "\nimport shiny\n" and path.name.endswith("_main.py"):
+                # skip shiny/_main.py file
+                continue
+
+            if search_txt in file_txt:
+
+                for i, line in enumerate(file_txt.split("\n")):
+                    if line.startswith(search_txt.strip()):
+                        # bad_entries.append((path.relative_to(root_here), i + 1, line))
+                        bad_entries.append(
+                            BadLine(path.relative_to(root_here), i + 1, line)
+                        )
+
+    if len(bad_entries) > 0:
+        print("Bad entries found:")
+        for entry in bad_entries:
+            print(entry)
+    # Ensure no bad entries exist
+    assert (
+        len(bad_entries) == 0
+    ), "Unexpected shiny files containing `from shiny.FOO import BAR`, `from shiny import FOO`, or `import shiny`"