Better error for mypy -p package without py.typed (python#17908)

Improve the error message when running `mypy -p packagename` with a
package that doesn't have py.typed set.

> package 'example' is skipping due to missing py.typed marker. See
https://mypy.readthedocs.io/en/stable/installed_packages.html for more
details

Fix python#17048

---------

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/main.py

@@ -22,7 +22,14 @@
 from mypy.errors import CompileError
 from mypy.find_sources import InvalidSourceList, create_source_list
 from mypy.fscache import FileSystemCache
-from mypy.modulefinder import BuildSource, FindModuleCache, SearchPaths, get_search_dirs, mypy_path
+from mypy.modulefinder import (
+    BuildSource,
+    FindModuleCache,
+    ModuleNotFoundReason,
+    SearchPaths,
+    get_search_dirs,
+    mypy_path,
+)
 from mypy.options import INCOMPLETE_FEATURES, BuildType, Options
 from mypy.split_namespace import SplitNamespace
 from mypy.version import __version__
@@ -1413,7 +1420,15 @@ def set_strict_flags() -> None:
                 fail(f"Package name '{p}' cannot have a slash in it.", stderr, options)
             p_targets = cache.find_modules_recursive(p)
             if not p_targets:
-                fail(f"Can't find package '{p}'", stderr, options)
+                reason = cache.find_module(p)
+                if reason is ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS:
+                    fail(
+                        f"Package '{p}' cannot be type checked due to missing py.typed marker. See https://mypy.readthedocs.io/en/stable/installed_packages.html for more details",
+                        stderr,
+                        options,
+                    )
+                else:
+                    fail(f"Can't find package '{p}'", stderr, options)
             targets.extend(p_targets)
         for m in special_opts.modules:
             targets.append(BuildSource(None, m, None))