Scope integration discovery to custom_components

[kingpanther13]

Proposed change

scripts/helpers/integration_path.py locates a submission's integration by
walking the entire clone for *manifest.json and exiting with "No manifest"
unless exactly one exists. The publishing documentation scopes the structure
rule to ROOT_OF_THE_REPO/custom_components/ ("there can only be one
subdirectory to ROOT_OF_THE_REPO/custom_components/"), so a repository that
satisfies the documented rule but also ships other legitimately-named
manifest.json files (a Supervisor add-on's inner component, test fixtures)
fails the Hassfest check before hassfest runs. Reproducer: #8966 - one
integration under custom_components/, plus add-on and fixture manifests
elsewhere; the HACS action itself passes on that repository.

This change scopes the search to ROOT/custom_components/ when that
directory exists and keeps the current whole-clone behavior otherwise
(content_in_root repositories are unaffected). Repositories with multiple
integrations under custom_components/ still fail exactly as today. Verified
against both shapes locally.

If the whole-clone uniqueness is intentional, happy to close this - guidance
on how monorepo-shaped repositories (integration + Supervisor add-on in one
repo) should submit would then be appreciated.