feat: add support for layered package cache

[kelvinou01]

Description

Resolves #43

Stuff to note

• I tried to make this not a break, but alas. Specifically, i made PackageCacheError a #[non_exhaustive] enum, and modified some variants.

Old PR for ref

[wolfv]

Hi @kelvinou01 - I also started hacking on this a bit but it looks like you are already much further progressed :)

Here was my attempt: #998

[gzm55]

@kelvinou01 is this pr ready?

[gzm55]

it seems the pr does not contains the changes of the config key.

[kelvinou01]

@kelvinou01 is this pr ready?

@gzm55 not yet, is there an urgent need for it?

[gzm55]

no urgent need. but expect this feature for multi-user environment

[kelvinou01]

@wolfv hey, could you help me review this?

Also, I did try implementing storing the capabilities cache like #998. However, I thought this PR was large enough already, so the capabilities cache can be introduced in a subsequent PR.

[gzm55]

when select a read or write cache, can we prefer the one from which we can make a hard link?

[kelvinou01]

@gzm55 I see how that can be useful! Do you think this feature is necessary to introduce with this PR, or can we do it in a subsequent one as i mentioned above?

[gzm55]

@gzm55 I see how that can be useful! Do you think this feature is necessary to introduce with this PR, or can we do it in a subsequent one as i mentioned above?

I think a subsequent pr for the feature mentioned above is ok.

[gzm55]

@baszalmstra is there other suggestions about this pr?

[baszalmstra]

This looks good to me. Would you be able to fix the merge conflict? We can merge afterwards unless @wolfv has specific comments.

[kelvinou01]

Would you be able to fix the merge conflict

@baszalmstra done!

[baszalmstra]

@wolfv Would you be able to take a look at this PR?