Faster GC `--keep-derivations` without complicating the `Derivation` data type

[Ericson2314]

Is your feature request related to a problem?

GC with derivation scanning is currently slow. If we always parse structured attrs JSON, it will be even slower.

Proposed solution

The simplest thing to do is make structured attrs a "lazily loaded field", so we can defer parsing until its is needed. but that is gross.

Alternative solutions

Better solutions are:

• Use a special derivation parser for GC that just gets out the output paths, since those are the only things which are needed. That would be faster than than parsing the whole derivation with or without structured attrs parsed JSON.

• Stop using .drv files, and instead use database tables. Than we can look at e.g. a DerivationOutputs table with high throughput.

Additional context

#13263

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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[Mic92]

Basically our reference scan that do after a build.

nix/src/libstore/gc.cc

Line 767 in b062730

for (auto & [name, maybeOutPath] : queryPartialDerivationOutputMap(*path))

[Ericson2314]

Oh jokes on me, see LocalStore::queryStaticPartialDerivationOutputMap, we already optimize just this one case with a SQLite table! We've done this since c4d388a, evidentally for expressly the purpose of GC.

[Ericson2314]

Turns out this is only an issue with floating content-addressing derivations. Will make a separate issue for that.