Generate supplemental Debian packages

[cpuguy83]

Adds Debian control stanzas and package-specific install, maintainer, systemd, and permissions behavior, with unit and LLB-level coverage.

This is layer 3 of the native supplemental-packages stack and depends on #1147.

Related to #607.

[Copilot]

Pull request overview

Adds Debian-side support for “supplemental packages” (subpackage/binary package stanzas) so a single Dalec build can emit multiple Debian packages, aligning with the broader supplemental-packages stack (related to #607, depends on #1147).

Changes:

• Extend Debian control generation to emit additional binary package stanzas for target subpackages.
• Generate per-subpackage Debian install scripts and maintainer scripts (postinst) and handle systemd units per owning package.
• Add unit-level and LLB-definition-level tests covering subpackage control stanzas, install file generation, postinst generation, perms/systemd behavior, and ordering.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packaging/linux/deb/templates/debian_control.tmpl	Emits appended subpackage stanzas into the generated debian/control.
packaging/linux/deb/template_control.go	Implements subpackage stanza rendering (Depends/Recommends/Provides/Conflicts/Replaces/Description).
packaging/linux/deb/template_control_test.go	Unit tests for control output with subpackages (naming, sorting, deps, auto-requires behavior).
packaging/linux/deb/template_rules.go	Routes systemd override behavior to the correct package (primary vs subpackage) and checks subpackage perms.
packaging/linux/deb/template_rules_test.go	Adds coverage ensuring custom systemd enable snippets land in subpackage postinst when needed.
packaging/linux/deb/template_custom_dh_installsystemd_postinst.go	Generates custom systemd enable/start snippets per owning package (primary + subpackages).
packaging/linux/deb/debroot.go	Generates debroot artifacts for subpackages: .install files, .links, .docs, postinst, and per-package perms/systemd partials.
packaging/linux/deb/debroot_subpackage_test.go	LLB-level tests validating that subpackage artifacts produce the expected debroot files and rules behavior.

Comments suppressed due to low confidence (1)

packaging/linux/deb/debroot.go:280

• dir values passed into checkAndWritePerms are absolute paths (e.g. /usr/bin, /usr/share/doc/...). Using filepath.Join(basePath, dir, ...) drops basePath when dir is absolute, so fix_perms.sh will chmod host paths instead of files under debian/<pkgName>/....

			p := filepath.Join(basePath, dir, resolvedName)
			// TODO: do i need this?
			if cfg.SubPath != "" {
				p = filepath.Join(basePath, dir, cfg.SubPath, resolvedName)
			}

[invidian]

Some review comments, although this is not thorough given the amount of code in this PR. I also don't know what testing level we cover here, but perhaps a lot is missing. Given that this could is large and complex, it would be nice to have a robust test suite for it so we can at least attempt to refactor it in the future.