apk/installed: parse Z: (file checksum) token in ParseInstalled

[Dev-X25874]

AddInstalledPackage writes a Z:<checksum> line for every regular
file that carries a checksum, and real installed databases are full of
them. ParseInstalled had no case "Z": handler, so those lines were
silently dropped on read-back — every reconstructed tar.Header came
back with a nil PAXRecords map and no checksum.

Add the missing case, mirroring the write path: allocate PAXRecords
on demand and store the value under paxRecordsChecksumKey, the same
key used by installRegularFile and AddInstalledPackage.

[Dev-X25874]

Hey @a-crate — would you mind taking a look at this when you get a chance? This adds the missing Z: case in ParseInstalled to handle file checksums that were being silently dropped on read-back. Happy to answer any questions or make changes based on your feedback!

[a-crate]

Sorry for delay. Looking.

[a-crate]

Z: is an APK V2 field: https://wiki.alpinelinux.org/wiki/Apk_spec

apko doesn't have support for APK V2 yet, and while it would be good to add it I don't think adding piecemeal support for individual APK V2 fields is the right way go about this. Even if we parse APK V2 fields in the installed DB we're not going to write new correct entries in V2 format.

Do you have a specific usecase that's broken without this change?

[a-crate]

Unify the indentation here please, keep all cases statements at the same indentation and don't modify indentation from 341:344. gofmt should do this for you, I think. The diff and control flow is a bit confusing otherwise.