chore(asm): less appsec imports (#8057)

For now `ddtrace.appsec._constants` does not depend of other appsec
modules. So importing it is lightweight.

By moving one import in `ddtrace/appsec/_utils.py` from global to local
scope, this allows `ddtrace.appsec._utils` and
`ddtrace.appsec._capabilities` to also not depend of all other appsec
modules, ensuring less memory usage when asm stays disabled.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] 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)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Author: christophe-papazian

ddtrace/appsec/_utils.py

@@ -1,7 +1,6 @@
 import os
 import uuid
 
-from ddtrace.appsec import _asm_request_context
 from ddtrace.appsec._constants import API_SECURITY
 from ddtrace.constants import APPSEC_ENV
 from ddtrace.internal.logger import get_logger
@@ -20,6 +19,7 @@ def parse_response_body(raw_body):
 
     import xmltodict
 
+    from ddtrace.appsec import _asm_request_context
     from ddtrace.appsec._constants import SPAN_DATA_NAMES
     from ddtrace.contrib.trace_utils import _get_header_value_case_insensitive