chore(iast): fix circular imports with importlib_metadata [backport 2.21] (#12728)

Backport 3c9f4ed from #12711 to 2.21.

If the IAST hook for importing modules is triggered before Telemetry
(the other product that calls this function), a circular import can
occur when calling `get_package_distributions` since it relies on
`importlib_metadata`.

To prevent this, since `get_package_distributions` caches the data, any
subsequent calls won't invoke `importlib_metadata` again. For this
reason, we execute this function before IAST runs.

Error example

https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/844601482
```
Traceback (most recent call last):
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/ddtrace/internal/module.py", line 257, in _find_first_hook
    callable(cond) and cond(module.__name__)
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/ddtrace/appsec/_iast/_ast/ast_patching.py", line 484, in _should_iast_patch
    if _is_first_party(module_name):
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/ddtrace/appsec/_iast/_ast/ast_patching.py", line 465, in _is_first_party
    _IMPORTLIB_PACKAGES = set(get_package_distributions())
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/ddtrace/internal/packages.py", line 63, in get_package_distributions
    _PACKAGE_DISTRIBUTIONS = _packages_distributions()
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/ddtrace/internal/packages.py", line 325, in _packages_distributions
    pkg_to_dist[pkg].append(dist.metadata["Name"])
  File "/app/benchmarks/.venv_baseline/lib/python3.9/site-packages/importlib_metadata/__init__.py", line 499, in metadata
    return _adapters.Message(email.message_from_string(text))
AttributeError: partially initialized module 'importlib_metadata._adapters' has no attribute 'Message' (most likely due to a circular import)
```

This PR is a cherry-pick of one of the commits of this PR
#12639

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Alberto Vara <[email protected]>

Author: github-actions[bot]

ddtrace/appsec/_iast/__init__.py

@@ -92,6 +92,12 @@ def enable_iast_propagation():
     global _iast_propagation_enabled
     if _iast_propagation_enabled:
         return
+    # We need to preload the package_distributions because if we call importlib_metadata inside a module hook
+    # it raises a circular-import
+    from ddtrace.internal.packages import get_package_distributions
+
+    get_package_distributions()
+
     log.debug("IAST enabled")
     ModuleWatchdog.register_pre_exec_module_hook(_should_iast_patch, _exec_iast_patched_module)
     _iast_propagation_enabled = True