all: Rework page store, add a dynacache to enable bigger data/content, and some general spring cleaning

[bep]

This PR is big. I could/should probably try to split it, but the pieces are in many ways related, and splitting it would be hard.

There are some breaking changes in this commit, see #11455.

Closes #11455
Closes #11549

What Changed

This fixes a set of bugs (see issue list) and it is also paying some technical debt accumulated over the years. We now build with Staticcheck enabled in the CI build.

The performance should be about the same as before for regular sized Hugo sites, but it should perform and scale much better to larger data sets, as objects that uses lots of memory (e.g. rendered Markdown, big JSON files read into maps with transform.Unmarshal etc.) will now get automatically garbage collected if needed. Performance on partial rebuilds when running the server in fast render mode should be the same, but the change detection should be much more accurate.

A list of the notable new features:

1. A new dependency tracker that covers (almost) all of Hugo's API and is used to do fine grained partial rebuilds when running the server.
2. A new and simpler tree document store which allows fast lookups and prefix-walking in all dimensions (e.g. language) concurrently.
3. You can now configure an upper memory limit allowing for much larger data sets and/or running on lower specced PCs.
4. We have lifted the "no resources in sub folders" restriction for branch bundles (e.g. sections).

Memory Limit

Hugos will, by default, set aside a quarter of the total system memory, but you can set this via the OS environment variable HUGO_MEMORYLIMIT (in gigabytes). This is backed by a partitioned LRU cache used throughout Hugo. A cache that gets dynamically resized in low memory situations, allowing Go's Garbage Collector to free the memory.

Dependency Tracker

Hugo has had a rule based coarse grained approach to server rebuilds that has worked mostly pretty well, but there have been some surprises (e.g. stale content). This is now revamped with a new dependency tracker that can quickly calculate the delta given a changed resource (e.g. a content file, template, JS file etc.). This handles transitive relations, e.g. $page -> js.Build -> JS import, or $page1.Content -> render hook -> site.GetPage -> $page2.Title, or $page1.Content -> shortcode -> partial -> site.RegularPages -> $page2.Content -> shortcode ..., and should also handle changes to aggregated values (e.g. site.Lastmod) effectively.

This covers all of Hugo's API with 2 known exceptions (a list that may not be fully exhaustive):

1. Changes to files loaded with template func os.ReadFile may not be handled correctly. We recommend loading resources with resources.Get
2. Changes to Hugo objects (e.g. Page) passed in the template context to lang.Translate may not be detected correctly. We recommend having simple i18n templates without too much data context passed in other than simple types such as strings and numbers.

Note that the cachebuster configuration (when A changes then rebuild B) works well with the above, but we recommend that you revise that configuration, as it in most situations should not be needed. One example where it is still needed is with TailwindCSS and using changes to hugo_stats.json to trigger new CSS rebuilds.

Document Store

Previously, a little simplified, we split the document store (where we store pages and resources) in a tree per language. This worked pretty well, but the structure made some operations harder than they needed to be. We have now restructured it into one Radix tree for all languages. Internally the language is considered to be a dimension of that tree, and the tree can be viewed in all dimensions concurrently. This makes some operations re. language simpler (e.g. finding translations is just a slice range), but the idea is that it should also be relatively inexpensive to add more dimensions if needed (e.g. role).

Closes #11455
Closes #11549

Fixes #10104
Fixes #10380
Fixes #10694
Fixes #11439
Fixes #11453
Fixes #11457
Fixes #11466
Fixes #11540
Fixes #11551
Fixes #11556
Fixes #11654
Fixes #11661
Fixes #11663
Fixes #11664
Fixes #11669
Fixes #11671
Fixes #11807
Fixes #11808
Fixes #11809
Fixes #11815
Fixes #11840
Fixes #11860
Fixes #11883
Fixes #7425
Fixes #7436
Fixes #7437
Fixes #7544
Fixes #7882
Fixes #8307
Fixes #8498
Fixes #8927
Fixes #9192
Fixes #9324
Fixes #9343

[bep]

@jmooring I will let this PR sit as is over the weekend with the goal to merge it into the main branch early next week. This will obviously not be released right away (and even then probably a beta or 2), so there are plenty of chances to fix things/make adjustments before it goes live.

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