refactor(processor): replace filepath.Glob with doublestar.Glob for enhanced pattern matching

[BraveY]

• Replaced filepath.Glob with doublestar.Glob to support advanced glob patterns (e.g., **/*.py for recursive matching).

[gaius-qi]

LGTM

[chlins]

Currently, the glob path in the standard library is based on the standard implementation, and most language implementations are relatively uniform. While doublestar introduces convenience, it also introduces additional interpretation costs for users. So let's hold off for now and merge when there are similar requirements.

[aftersnow]

Status check on the current head (ea5b98e) against today's main. Two facts have changed since the "hold off" decision in March 2025:

1. doublestar is already a dependency of main. go.mod has github.com/bmatcuk/doublestar/v4 v4.10.0, pulled in by #473, and modelfile generate --include/--exclude match with doublestar semantics. The "extra interpretation cost for users" argument now cuts the other way: the two commands disagree on what a pattern means.

2. build silently drops ** patterns today. I built this Modelfile on main (f241cec):

NAME ws99
MODEL w.safetensors
CODE src/**/*.py

with src/y.py and src/a/b/x.py present. The build succeeded and only the MODEL layer was produced; filepath.Glob treats ** as *, matched nothing, and processor/base.go does not warn when a wildcard pattern matches zero files. A user who writes ** because generate --include accepts it loses files without any error.

So I think this change is worth reviving, with three adjustments:

• Rebase onto main. The module path is now github.com/modelpack/modctl, go.mod/go.sum and pkg/backend/processor/base.go all conflict, and base.go now has a literal-path branch (absolute paths, "file specified in Modelfile does not exist") that must stay as is; only the wildcard branch should switch to doublestar.Glob(os.DirFS(absWorkDir), pattern).
• Fail or warn when a wildcard pattern matches nothing. That is the actual footgun above, independent of which glob library is used.
• Add a test with a ** pattern in pkg/backend/processor.

cc @chlins since the earlier decision was yours.