Validate whether the module name is installed in the current environment

Closes #959

shiny/_custom_component_template_questions.py

@@ -1,10 +1,27 @@
 import re
+from importlib import util
 from pathlib import Path
 
 from prompt_toolkit.document import Document
 from questionary import ValidationError, Validator
 
 
+def is_existing_module(name: str) -> bool:
+    """
+    Check if a module name can be imported, which indicates that it is either
+    a standard module name, or the name of an installed module.
+    In either case the new module would probably cause a name conflict.
+    """
+    try:
+        spec = util.find_spec(name)
+        if spec is not None:
+            return True
+        else:
+            return False
+    except ImportError:
+        return False
+
+
 def is_pep508_identifier(name: str):
     """
     Checks if a package name is a PEP 508 identifier.
@@ -65,6 +82,14 @@ def validate(self, document: Document):
                 cursor_position=len(name),
             )
 
+        # Using the name of an existing package causes an import error
+
+        if is_existing_module(name):
+            raise ValidationError(
+                message="Package already installed in your current environment.",
+                cursor_position=len(name),
+            )
+
 
 def update_component_name_in_template(template_dir: Path, new_component_name: str):
     """