…nsistencies (#48279)

### What does this PR do?
Remove `gazelle` exclusions that had to be put in place pending a fix for `gazelle` confused by `build` directories on "broken" case-insensitive filesystem stacks (docker/desktop-feedback#251).
Doing so, I realized there were inconsistencies in the `gazelle` setup and took the opportunity to fix them.

### Motivation
#48455 indeed allows to get rid of the following exclusions:
```
# TODO(agent-build): This exclusion is ready for removal, blocked by issues caused by the interaction of
# Bazel with directories named `build` created by the CMake builds. We can remove this once we remove CMake
# support or we upgrade to a Bazel version containing a fix: bazelbuild/bazel#26842
# In the meantime, temporarily remove this exclusion locally if regeneration of BUILD files is required
# gazelle:exclude rtloader
...
# gazelle:exclude .gitlab
```

Nearby inconsistencies in the `gazelle` setup:
- `build_tags` is passed by attribute (`build_tags = ALL_BUILD_TAGS`) => OK (**canonical**),
- `external` was passed by `args` (`-external=static`) => pass by attribute (`external = "static"`),
- `prefix` was passed by directive (`# gazelle:prefix github.com/DataDog/datadog-agent`) => pass by attribute (`prefix = "github.com/DataDog/datadog-agent"`),
- `args` happens to be a [compatibility shim](bazel-contrib/bazel-gazelle#62) => use `extra_args` (**canonical**) when there's no corresponding attribute yet:
   - `build_file_name` was passed by `args` and doesn't have a corresponding attribute yet => pass by `extra_args`,
   - `strict` was passed from a single GitLab job and doesn't have a corresponding attribute yet => pass by `extra_args`.

### Describe how you validated your changes
Kind assistance of @alopezz and/or @JSGette who run Linux containers on their macOS-powered laptop.

Co-authored-by: regis.desgroppes <[email protected]>