jit package fetching

[paperdave]

see #19063. this makes zig fetch compile and execute lib/compiler/fetch.zig

the following refactors were made:

• src.introspect.isUpDir is now std.fs.path.isUpDir.
• src.introspect is now std.zig.introspect
• src.main's findTemplates, Templates, findBuildRoot, BuildRoot, and FindBuildRootOptions are all moved into std.zig.introspect
• src.Package's Fetch, Manifest and build_zig_basename are moved into std.zig.package. this container name is lowercase as it has no fields.

src.Package still exists. i found moving Module nontrivial as it depends on Compilation.Config (which depends on build_options) and Zcu.File. the compromise is an ?*anyopaque on Fetch's .module field. it seems only two pointer casts are needed to make this compile.

[andrewrk]

I played with this locally but it didn't actually save any time building the compiler because the main fetching code, which is n cmdBuild, is still present:

Benchmark 1 (4 runs): before/zig build-exe -fno-llvm -fno-lld ...
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          12.8s  ± 61.5ms    12.7s  … 12.8s           0 ( 0%)        0%
  peak_rss            346MB ±  459KB     345MB …  346MB          0 ( 0%)        0%
  cpu_cycles         64.1G  ±  148M     63.9G  … 64.2G           0 ( 0%)        0%
  instructions        129G  ±  350K      129G  …  129G           0 ( 0%)        0%
  cache_references   4.07G  ± 14.7M     4.06G  … 4.09G           0 ( 0%)        0%
  cache_misses        484M  ± 2.92M      482M  …  488M           0 ( 0%)        0%
  branch_misses       446M  ± 1.90M      444M  …  448M           0 ( 0%)        0%
Benchmark 2 (4 runs): after/zig build-exe -fno-llvm -fno-lld ...
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          12.7s  ±  111ms    12.7s  … 12.9s           0 ( 0%)          -  0.4% ±  1.2%
  peak_rss            346MB ±  141KB     345MB …  346MB          0 ( 0%)          +  0.0% ±  0.2%
  cpu_cycles         63.8G  ± 85.2M     63.7G  … 63.9G           0 ( 0%)          -  0.4% ±  0.3%
  instructions        129G  ±  691K      129G  …  129G           0 ( 0%)          -  0.3% ±  0.0%
  cache_references   4.04G  ± 12.6M     4.02G  … 4.05G           0 ( 0%)          -  0.8% ±  0.6%
  cache_misses        488M  ± 1.38M      487M  …  490M           0 ( 0%)          +  0.9% ±  0.8%
  branch_misses       444M  ±  962K      443M  …  445M           0 ( 0%)          -  0.4% ±  0.6%

I don't think it makes sense to extract only zig fetch to be built externally when the main fetching logic is still bundled into the compiler. It should be both, or neither.