Add parallel build support with -parallel and -jobs options

[arthurbdiniz]

This commit introduces parallel package building to ratt.

• New flag -parallel enables parallel builds.
• New flag -jobs controls the number of concurrent workers (default: number of CPU cores, must be > 0).
• Added buildPackagesSequential and buildPackagesParallel helpers.
• Refactored main build loop to support sequential or parallel execution.

This allows faster builds by utilizing multiple CPU cores.

---

Example:

~/go/bin/ratt -parallel golang-github-go-logr-zapr_1.3.0-1_amd64.changes
2025/09/12 17:09:43 Loading changes file "golang-github-go-logr-zapr_1.3.0-1_amd64.changes"
2025/09/12 17:09:43  - 1 binary packages: golang-github-go-logr-zapr-dev
2025/09/12 17:09:43 Corresponding .debs (will be injected when building):
2025/09/12 17:09:43     golang-github-go-logr-zapr-dev_1.3.0-1_all.deb
2025/09/12 17:09:43 Setting -dist=sid (from .changes file)
2025/09/12 17:09:43 Figuring out reverse build dependencies using dose-ceve(1). This might take a while
2025/09/12 17:10:14 Found 56 reverse build dependencies
2025/09/12 17:10:14 Setting -sbuild_dist=unstable (from .changes file)
2025/09/12 17:10:14 Building packages in parallel using 6 workers
2025/09/12 17:10:14 Building package 1 of 56: golang-github-containerd-stargz-snapshotter
2025/09/12 17:10:14 Building package 2 of 56: sigstore-go
2025/09/12 17:10:14 Building package 3 of 56: golang-github-containers-common
2025/09/12 17:10:14 Building package 4 of 56: golang-github-containerd-nydus-snapshotter
2025/09/12 17:10:14 Building package 4 of 56: golang-github-openshift-imagebuilder
2025/09/12 17:10:14 Building package 6 of 56: docker-compose

[ottok]

@arthurbdiniz Did you notice you got feedback on this? There are no comments from you so it is a bit unclear, but you did push changes.

Perhaps you want to comment explicitly asking Michael to re-review, or press the arrow button thing?

[arthurbdiniz]

Pull Request updated, waiting for another review.

[stapelberg]

add defer resultsMu.Unlock() after this line and remove the explicit resultsMu.Unlock() below. It’s good style to defer the unlocking so that one can’t forget it in any code path below.