fix: forward --pathmap: option to FCS so CallerFilePath respects (PathMap)

[fable-repo-assist]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Summary

Fixes #4061 — [<CallerFilePath>] always produces absolute paths even when the project configures <PathMap>, <DeterministicSourcePaths>, or <SourceRoot>.

Root Cause

extractUsefulOptionsAndSources in ProjectCracker.fs selectively forwards only a small set of compiler options to FCS (--langversion:, --checknulls+, --nowarn, --warnon, --define:). The --pathmap: flag emitted by MSBuild (from <PathMap> / <DeterministicSourcePaths>) was silently dropped, leaving g.pathMap empty in FCS. Since PathMap.apply is a no-op on an empty map, [<CallerFilePath>] always returned the full absolute path.

Fix

Forward --pathmap: from the main project only — mirroring how --langversion:preview is handled — so that FCS applies the configured path mapping when resolving CallerFilePath arguments:

// Forward pathmap from the main project so CallerFilePath respects <PathMap>/<DeterministicSourcePaths>
elif line.StartsWith("--pathmap:", StringComparison.Ordinal) && isMainProj then
    accSources, line :: accOptions

Restricting this to isMainProj means path mappings from referenced libraries don't bleed into the compilation of the main project (consistent with how other project-level options are handled).

Testing

This is a project-cracker-level change. The fix takes effect as soon as the fsproj contains the relevant MSBuild properties, e.g.:

<PropertyGroup>
  <Deterministic>true</Deterministic>
  <DeterministicSourcePaths>true</DeterministicSourcePaths>
  <PathMap>$(MSBuildThisFileDirectory)=.</PathMap>
</PropertyGroup>

With this change, [<CallerFilePath>] should return ./Api/Program.fs instead of the full absolute path.

Generated by 🌈 Repo Assist, see workflow run. Learn more.

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@51c8f6ad4357d2ecc06e47120031b3d75e80227d