fix(ci_visibility): do not use is_user_code [backport 3.16] (#14836)

Backport 7cd4d9c from #14794 to 3.16.

## Description

Currently, in Test Impact Analysis code coverage, we use
`is_user_code()` to decide whether to instrument a module. This function
was conceived for use in non-CI contexts where user code may be
installed in site-packages like external dependencies. This is not
necessary in Test Optimization, where we only want to cover code inside
the current repo (TIA only works with filenames that are relative to the
repo root, and we already set the repo root as the include path of
`ModuleCodeCollector`). Removing this check also resolves a bug where
test packages are incorrectly detected as third-party if a third-party
package contains files under `test/*` or `tests/*` that get installed at
site-packages level (`sumy` and `sendgrid` do this, for instance).

## Testing

Unit tests + manual testing in dogweb.

## Risks

Potentially covering more code than expected. Should not be an issue
because we already exclude site-packages and similar
[here](https://github.com/DataDog/dd-trace-py/blob/vitor-de-araujo%2FSDTEST-2747%2Fcoverage-third-party/ddtrace/internal/coverage/code.py#L53).

## Additional Notes

None.

Author: vitor-de-araujo

ddtrace/internal/coverage/code.py

@@ -14,7 +14,6 @@
 from ddtrace.internal.coverage.util import collapse_ranges
 from ddtrace.internal.logger import get_logger
 from ddtrace.internal.module import ModuleWatchdog
-from ddtrace.internal.packages import is_user_code
 from ddtrace.internal.packages import platlib_path
 from ddtrace.internal.packages import platstdlib_path
 from ddtrace.internal.packages import purelib_path
@@ -336,9 +335,6 @@ def transform(self, code: CodeType, _module: ModuleType) -> CodeType:
             # Don't instrument code from standard library/site packages/etc.
             return code
 
-        if not is_user_code(code_path):
-            return code
-
         retval = self.instrument_code(code, _module.__package__ if _module is not None else "")
 
         if self._collect_import_coverage:

releasenotes/notes/ci_visibility-fix-itr-user-code-61c09671abe6df17.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    CI Visibility: This fix resolves an issue where tests would be incorrectly detected as third-party code if a
+    third-party package containing a folder with the same name as the tests folder was installed. For instance, the
+    ``sumy`` package installs files under ``tests/*`` in ``site-packages``, and this would cause any modules under
+    ``tests.*`` to be considered third-party.