Give autoschedulers their own CMake package

[alexreinking]

HalideCompilerConfigVersion.cmake correctly rejects a mismatched-bitness install, since Halide::Generator/Halide::PyStubs are real linked host binaries. But that check uses CMAKE_SIZEOF_VOID_P, which reflects the target ABI while cross-compiling, not the real host running CMake -- so add_halide_library(... AUTOSCHEDULER ...)'s find_package(HalideCompiler) spuriously fails whenever host and target word sizes differ (e.g. a 64-bit host cross-compiling for a 32-bit bare-metal target).

Autoscheduler plugins are dlopen()'d by the generator process at Generator run time and never linked, so nothing about their compiled ABI is exposed to or constrained by the consuming project. Split them into their own HalideAutoschedulers package, marked ARCH_INDEPENDENT -- a deliberate, documented (ab)use of the flag that doesn't apply to HalideCompiler's other (actually linked) targets.

Breaking changes

None

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Commits include AI attribution where applicable (see Code of Conduct)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.95%. Comparing base (aa169dc) to head (4ffe611).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9292      +/-   ##
==========================================
- Coverage   70.00%   69.95%   -0.05%     
==========================================
  Files         258      258              
  Lines       77353    77353              
  Branches    18837    18837              
==========================================
- Hits        54151    54114      -37     
- Misses      17626    17643      +17     
- Partials     5576     5596      +20     

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