Special case warning for requirements.txt install

Added a special warning for users who accidentally do
`pip install requirements.txt` and forget the `-r` flag.
Issue pypa#9908

src/pip/_internal/resolution/resolvelib/factory.py

@@ -540,6 +540,13 @@ def _report_single_requirement_conflict(self, req, parent):
             req_disp,
             ", ".join(versions) or "none",
         )
+        if str(req) == "requirements.txt":
+            logger.critical(
+                "HINT: You are attempting to install a package literally "
+                'named "requirements.txt" (which cannot exist). Consider '
+                "using the '-r' flag to install the packages listed in "
+                "requirements.txt"
+            )
 
         return DistributionNotFound(f"No matching distribution found for {req}")
 

tests/functional/test_install_extras.py

@@ -143,6 +143,12 @@ def test_install_special_extra(script):
     ) in result.stderr, str(result)
 
 
+def test_install_requirements_no_r_flag(script):
+    '''Beginners sometimes forget the -r and this leads to confusion'''
+    result = script.pip('install', 'requirements.txt', expect_error=True)
+    assert 'literally named "requirements.txt"' in result.stderr
+
+
 @pytest.mark.parametrize(
     "extra_to_install, simple_version", [
         ['', '3.0'],