Always clone submodules for new source resolvers

[smoparth]

Context

The design proposal (docs/proposals/new-resolver-config.md) states under "git clone":

Like pip's VCS feature, all git clone operations automatically retrieve all submodules recursively.

And under "Deprecations":

git_options.submodules is not needed. Like pip, Fromager will always clone all submodules.

Parent epic: #936
Parent issue: #1048

Problem

Currently download_git_source() in sources.py reads git_opts.submodules and git_opts.submodule_paths to decide whether to clone submodules:

submodules: bool | list[str] = False
if git_opts.submodule_paths:
    submodules = git_opts.submodule_paths
elif git_opts.submodules:
    submodules = True

This logic applies to all git clones, including those triggered by the new source config. Since submodules defaults to False, no submodules are cloned even when a new source resolver triggers the git clone.

For example, a package configured as:
source:
provider: gitlab-tag-git
project_url: https://gitlab.example/org/project
Would clone without submodules, even though the proposal says it should always clone them (like pip does).

Fix
When the git clone is triggered by a new source resolver (not req.url), always pass submodules=True to gitutils.git_clone() regardless of git_options.submodules. The old git_options.submodules field should only apply to legacy req.url flows.

One approach is to pass a flag to download_git_source() indicating the clone was triggered by a source resolver. Another is to check pbi.source_resolver inside the function.

Consider using gitutils.git_clone_fast() for source-resolver-triggered clones, since it already handles submodules automatically and uses efficient blobless cloning.