Improve content map, memory cache and dependency resolution

TODO(bep) improve commit message.

Hugo has always been a active user of in-memory caches, but before this commit we did nothing to control the memory usage.

One failing example would be loading lots of big JSON data files and unmarshal them via `transform.Unmarshal`.

This commit consolidates all these caches into one single LRU cache with an eviction strategy that also considers used vs. available memory.

Hugo will try to limit its memory usage to 1/4 or total system memory, but this can be controlled with the `HUGO_MEMORYLIMIT` environment variable (a float value representing Gigabytes).

A natural next step after this would be to use this cache for `.Content`.

Fixes gohugoio#8307
Fixes gohugoio#8498
Fixes gohugoio#8927
Fixes gohugoio#9192
Fixes gohugoio#9189
Fixes gohugoio#7425
Fixes gohugoio#7437
Fixes gohugoio#7436
Fixes gohugoio#7882
Updates gohugoio#7544
Fixes gohugoio#9224

Author: bep

bench.sh

@@ -1 +1 @@
-gobench -package=./hugolib -bench="BenchmarkSiteNew/Deep_content_tree"
+gobench --package ./hugolib --bench "BenchmarkSiteNew/Regular_Deep" -base v0.89.4