go.tools/go/loader: Add Program.FilePath convenience method for getting the full path of a source file.

LGTM=gri
R=gri, adonovan
CC=golang-codereviews
https://golang.org/cl/107160049

go/loader/loader.go

@@ -460,6 +460,16 @@ func (prog *Program) InitialPackages() []*PackageInfo {
 	return infos
 }
 
+// FilePath returns the full path to the specified file.  It returns the empty
+// string if the file does not belong to one of this Program's packages.
+//
+func (prog *Program) FilePath(file *ast.File) string {
+	if f := prog.Fset.File(file.Package); f != nil {
+		return f.Name()
+	}
+	return ""
+}
+
 // ---------- Implementation ----------
 
 // importer holds the working state of the algorithm.