Packages (pkgsrc): add dynamic prefix discovery

[Suyun114]

Summary

Previously pkgsrc detection hardcoded /usr/pkg/pkgdb, which missed installations at non-standard prefixes (e.g. /opt/pkg) and database directory layouts (e.g. .pkgdb).

Now ffPackagesGetPkgsrc finds the pkgin binary via PATH, derives the prefix, reads <prefix>/etc/mk.conf for PKG_DBDIR, and falls back to <prefix>/pkgdb. Analogous to ffPackagesGetNix, the function appends to baseDir for Bedrock Linux compatibility.

Support added for Linux, NetBSD, SunOS, and macOS.

Changes

• New src/detection/packages/packages_pkgsrc.c — shared helper
• packages_linux.c / packages_apple.c / packages_nbsd.c / packages_sunos.c — all four now call ffPackagesGetPkgsrc
• packages.h — declaration guarded
• CMakeLists.txt — includes the new file

Checklist

• I have tested my changes locally.

[codacy-production]

Up to standards ✅

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[CarterLi]

Cleaned a bit.

I'm pretty sure that this PR will break bedrock. As this PR searches pkgin in $PATH, if pkgin is installed in one of bedrock strat, it will prepend the bedrock strat prefix twice. For example:

• baseDir: /bedrock/strata/distro
• pkgin path found in $PATH: /bedrock/strata/distro/usr/bin/pkgin
• ffStrbufAppendF(baseDir, "%s/pkgdb", pkginPath.chars): /bedrock/strata/distro/bedrock/strata/distro/usr/pkgdb

In addition, how bedrock strat handles PKG_DBDIR is still unknown.

Unless this is properly tested in bedrock, I don't think I want to merge it.