fix: Exclude external directory when generating python report (#2136)

This PR will reduce the time it take to run `bazel coverage` 

*Before this fix,*

 ```
 bazel coverage  --cache_test_results=no  //...
 Elapsed time: 7.054s, Critical Path: 6.87s
 
 ```
 
[lcov --list
bazel-out/_coverage/_coverage_report.dat](https://github.com/user-attachments/files/16681828/lcov.log)


*After*
```
 bazel coverage  --cache_test_results=no  //...
Elapsed time: 2.474s, Critical Path: 1.18s


$ lcov --list bazel-out/_coverage/_coverage_report.dat 
Reading tracefile bazel-out/_coverage/_coverage_report.dat
                                          |Lines       |Functions  |Branches    
Filename                                  |Rate     Num|Rate    Num|Rate     Num
================================================================================
[/home/ewianda/.cache/bazel/_bazel_ewianda/da4b4cc49e0e621570c9e24d6f1eab95/execroot/_main/bazel-out/k8-fastbuild/bin/benchsci/ml/nlp/]
test_tokenizer_stage2_bootstrap.py        | 6.0%    250|    -     0|    -      0

[benchsci/]
devtools/python/pytest_helper.py          |90.5%     21|    -     0|    -      0
ml/nlp/test_tokenizer.py                  | 100%     13|    -     0|    -      0
ml/nlp/tokenizer.py                       |61.8%     76|    -     0|    -      0
================================================================================
                                    Total:|26.1%    360|    -     0|    -      0

```

Related to #1434

---------

Co-authored-by: aignas <[email protected]>

CHANGELOG.md

@@ -29,6 +29,8 @@ A brief description of the categories of changes:
 
 ### Fixed
 * (gazelle): Fix incorrect use of `t.Fatal`/`t.Fatalf` in tests.
+* (toolchain) Omit third-party python packages from coverage reports from
+  stage2 bootstrap template.
 
 ### Added
 * Nothing yet

python/private/stage2_bootstrap_template.py

@@ -364,6 +364,14 @@ def _maybe_collect_coverage(enable):
                 # Pipes can't be read back later, which can cause coverage to
                 # throw an error when trying to get its source code.
                 "/dev/fd/*",
+                # The mechanism for finding third-party packages in coverage-py
+                # only works for installed packages, not for runfiles. e.g:
+                #'$HOME/.local/lib/python3.10/site-packages',
+                # '/usr/lib/python',
+                # '/usr/lib/python3.10/site-packages',
+                # '/usr/local/lib/python3.10/dist-packages'
+                # see https://github.com/nedbat/coveragepy/blob/bfb0c708fdd8182b2a9f0fc403596693ef65e475/coverage/inorout.py#L153-L164
+                "*/external/*",
             ],
         )
         cov.start()