Make sure to only read existing files (#63)

In particular don't use ReadPackage on paths that may not even be there

CHANGES.md

@@ -1,5 +1,11 @@
 This file describes changes in the smallgrp package.
 
+# 1.5.4 (2024-MM-DD)
+
+  - Don't attempt to load non-existent source files (this was
+    harmless so far as it failed silently, but in future GAP
+    versions may result in a scary warning or even an error)
+
 # 1.5.3 (2023-05-16)
 
   - Fix `SmallGroupsAvailable(p^7)` to return `false` when p > 11

read.g

@@ -15,8 +15,17 @@ READ_SMALL_LIB := function()
     local i, s, LoadFunc;
 
     LoadFunc := path ->
-                    {args...} ->
-                    ReadPackage("smallgrp", Concatenation(path, "/", args[1]));
+                function(args...)
+                    local relpath, filename;
+                    relpath := Concatenation(path, "/", args[1]);
+                    filename:= Filename( DirectoriesPackageLibrary( "smallgrp", "" ), relpath );
+                    if filename <> fail and IsReadableFile( filename ) then
+                      Read( filename );
+                      return true;
+                    else
+                      return false;
+                    fi;
+                end;
     s := 1;
     repeat
         s := s + 1;