Resolve features from @(ReferencePath) after ResolveAssemblyReferences

[SimonCropp]

Feature detection now runs in a single PolyfillResolveFeatures target
hooked AfterTargets="ResolveAssemblyReferences" BeforeTargets="CoreCompile",
and inspects @(ReferencePath) — the final list of assemblies csc will see,
with NuGetPackageId / NuGetPackageVersion preserved on NuGet-resolved items.

Replaces the previous two-target arrangement (PreparePolyfill running early
off @(ResolvedCompileFileDefinitions), plus a DetectBclMemoryPackageRef
fallback off @(PackageReference)). The fallback was too optimistic: it
fired whenever a PackageReference for Microsoft.Bcl.Memory existed, even
when the dll was not actually in csc's /reference list (stale IDE state,
ExcludeAssets="compile", etc.). The result was that RefsBclMemory got set,
Polyfill's own Index/Range types were suppressed, and the BCL types were
also missing — producing CS0246 in Polyfill_IEnumerable_ElementAt and
Polyfill_IEnumerable_Take.

Inspecting @(ReferencePath) post-RAR makes the feature flags reflect what
csc will actually see, eliminating that whole class of bug.

Adds ConsumeBclMemoryExcludeCompile as a regression test that uses
ExcludeAssets="compile" to reproduce the failure mode without needing an
IDE in the loop. ConsumeBclMemoryNoResolve loses its now-impossible
PreparePolyfill override and stays as a plain smoke test.

If you want it shorter, the body can collapse to:

Single PolyfillResolveFeatures target runs after ResolveAssemblyReferences
and inspects @(ReferencePath), so feature flags only fire when the
corresponding assembly is actually in the reference list csc will see.

Replaces the early PreparePolyfill + DetectBclMemoryPackageRef pair, whose
@(PackageReference)-based fallback could set RefsBclMemory while the dll
was missing from /reference (stale IDE state, ExcludeAssets="compile"),
causing CS0246 in Polyfill_IEnumerable_ElementAt / Take.

Adds ConsumeBclMemoryExcludeCompile as a regression test.