Python: Add example of top-level module shadowing stdlib

Although this test is added under the `wrong` folder, the current results from
this CodeQL test is actually correct (compared with the Python
interpreter). However, they don't match what the extractor does when invoked
with `codeql database create`.

Since I deemed it "more than an easy fix" to change the extractor behavior for
`codeql database create` to match the real python behavior, and it turned out to
be quite a challenge to change the extractor behavior for all tests, I'm just
going to make THIS ONE test-case behave like the extractor will with `codeql
database create`...

This is a first commit, to show how the extractor works with qltest by default.

Inspired by the debugging in github#4640

Author: RasmusWL

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/LocalModuleWithRef.expected

@@ -0,0 +1,3 @@
+| Local module | cmd.py:0:0:0:0 | Module cmd | referenced in external file called | pdb.py |
+| Local module | cmd.py:0:0:0:0 | Module cmd | referenced in local file called | test_ok.py |
+| Local module | unique_name.py:0:0:0:0 | Module unique_name | referenced in local file called | unique_name_use.py |

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/LocalModuleWithRef.ql

@@ -0,0 +1,13 @@
+import python
+
+from ModuleValue mv, ControlFlowNode ref, string local_external
+where
+  ref = mv.getAReference() and
+  exists(mv.getScope().getFile().getRelativePath()) and
+  (
+    if exists(ref.getLocation().getFile().getRelativePath())
+    then local_external = "local"
+    else local_external = "external"
+  )
+select "Local module", mv, "referenced in " + local_external + " file called",
+  ref.getLocation().getFile().getShortName()

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/LocalModules.expected

@@ -0,0 +1,5 @@
+| cmd.py:0:0:0:0 | Module cmd |
+| test_fail.py:0:0:0:0 | Module test_fail |
+| test_ok.py:0:0:0:0 | Module test_ok |
+| unique_name.py:0:0:0:0 | Module unique_name |
+| unique_name_use.py:0:0:0:0 | Module unique_name_use |

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/LocalModules.ql

@@ -0,0 +1,5 @@
+import python
+
+from Module m
+where exists(m.getFile().getRelativePath())
+select m

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/ModuleWithLocalRef.expected

@@ -0,0 +1,3 @@
+| Module 'cmd' (local, not in stdlib, not missing) referenced in local file | test_ok.py:1 |
+| Module 'pdb' (external, in stdlib, not missing) referenced in local file | test_fail.py:3 |
+| Module 'unique_name' (local, not in stdlib, not missing) referenced in local file | unique_name_use.py:1 |

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/ModuleWithLocalRef.ql

@@ -0,0 +1,19 @@
+import python
+
+from ModuleValue mv, ControlFlowNode ref, string in_stdlib, string local_external, string is_missing
+where
+  ref = mv.getAReference() and
+  exists(ref.getLocation().getFile().getRelativePath()) and
+  (
+    if mv.getScope().getFile().inStdlib()
+    then in_stdlib = "in stdlib"
+    else in_stdlib = "not in stdlib"
+  ) and
+  (
+    if exists(mv.getScope().getFile().getRelativePath())
+    then local_external = "local"
+    else local_external = "external"
+  ) and
+  (if mv.isAbsent() then is_missing = "missing" else is_missing = "not missing")
+select "Module '" + mv.getName() + "' (" + local_external + ", " + in_stdlib + ", " + is_missing +
+    ") referenced in local file", ref.getLocation().toString()

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/README.md

@@ -0,0 +1,8 @@
+This test shows how we handle modules the shadow a module in the standard library.
+
+We manually replicate the behavior of `codeql database create --source-root <src-dir>`, which will use `-R <src-dir>`. By default, the way qltest invokes the extractor will cause different behavior. Therefore, we also need to move our code outside of the top-level folder, and it lives in `code/`.
+
+Because we have a `cmd.py` file, whenever the python interpreter sees `import cmd`, that is the file that will be used! --
+
+* `python test_ok.py` works as intended, and prints `Foo`
+* `python test_fail.py` raises an exception, since it imports `pdb.py` from the standard library, which (at least in Python 3.8) tries to import `cmd.py` from the standard library, but instead is served our `cmd.py` module. Therefore it fails with `AttributeError: module 'cmd' has no attribute 'Cmd'`

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/cmd.py

@@ -0,0 +1,2 @@
+foo = "Foo"
+print("my own cmd imported")

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/options

@@ -0,0 +1 @@
+semmle-extractor-options: --max-import-depth=2

python/ql/test/library-tests/PointsTo/regressions/wrong/module-imports/conflict-stdlib/test_fail.py

@@ -0,0 +1,3 @@
+# we import `pdb` which import the `cmd` module from the standard library
+# and allows us to set --max-import-depth=2, to make the test run fast
+import pdb