fix: forward --pathmap: option to FCS so CallerFilePath respects <PathMap>

When MSBuild generates compiler options for an fsproj that uses
<PathMap>, <DeterministicSourcePaths>, or <SourceRoot>, it emits a
--pathmap:... flag. Previously, extractUsefulOptionsAndSources discarded
this flag, so FCS always used an empty PathMap and CallerFilePath always
returned absolute paths regardless of the user's configuration.

Forward --pathmap: from the main project only (mirroring how --langversion:
is handled) so that [<CallerFilePath>] respects the configured path
mappings at compile time.

Closes #4061

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

src/Fable.Compiler/ProjectCracker.fs

@@ -438,6 +438,9 @@ let private extractUsefulOptionsAndSources
         // Only forward the nullness flag if it is coming from the main project
         elif line.StartsWith("--checknulls+", StringComparison.Ordinal) && isMainProj then
             accSources, line :: accOptions
+        // Forward pathmap from the main project so CallerFilePath respects <PathMap>/<DeterministicSourcePaths>
+        elif line.StartsWith("--pathmap:", StringComparison.Ordinal) && isMainProj then
+            accSources, line :: accOptions
         elif
             line.StartsWith("--nowarn", StringComparison.Ordinal)
             || line.StartsWith("--warnon", StringComparison.Ordinal)