Add PackageMeta_ autogenerated module (#11755)

[angerman]

Summary

Adds a new autogenerated module PackageMeta_<pkgname> that exposes build-environment metadata known at configure time. This complements the existing PackageInfo_<pkgname> (static .cabal metadata) and Paths_<pkgname> (install paths).

Closes #11755.
Related: #392, #826, #1506, #7355, #7488, #11258, #4746.

What PackageMeta_ provides

module PackageMeta_mypackage where

compiler        :: String   -- e.g. "ghc"
compilerVersion :: Version  -- e.g. Version [9,6,7] []
os              :: String   -- e.g. "linux", "darwin"
arch            :: String   -- e.g. "x86_64", "aarch64"
gitRevision     :: String   -- e.g. "abc123..." or ""
gitDirty        :: Bool     -- working tree dirty?
flag_debug      :: Bool     -- one Bool per cabal flag
flag_benchmarks :: Bool

Taxonomy

Module	Changes when...	Content
PackageInfo_	.cabal file edited	name, version, synopsis, copyright, homepage
PackageMeta_ (new)	Build environment changes	compiler, os, arch, flags, gitRevision
Paths_	Package installed to different prefix	bindir, libdir, datadir

Implementation

Follows the exact pattern established by PackageInfo_ (PR #8534):

• Distribution.Simple.Build.PackageMetaModule -- generation logic
• Distribution.Simple.Build.PackageMetaModule.Z -- Zinza-style renderer
• autogenPackageMetaModuleName added to BuildPaths
• Wired into builtinAutogenFiles in Build.hs
• Filtered from sdist in SrcDist.hs
• Test cases following the PackageMetaModule/ pattern

Note on git revision: generatePackageMetaModule accepts the git revision and dirty flag as parameters (since computing them requires IO, and builtinAutogenFiles is pure). This PR passes empty defaults ("", False). Wiring up the actual git rev-parse HEAD call at the appropriate IO call site is left for a follow-up, pending discussion on the issue about whether VCS info should be opt-in.

Test plan

• Verify PackageMetaModule.hs and Z.hs compile
• Verify generated module is valid Haskell (validated end-to-end with standalone renderer test)
• Verify cabal check warnings for missing autogen-modules
• Verify sdist filtering works
• Run existing test suite to check for regressions