Ticket-944: Make lookup of scala-library.jar and scala-compiler.jar more robust

This patch fixes a bug that causes `TemplateCompiler` to fail to load `scala-library.jar` and `scala-compiler.jar` on Windows 2003.
The corresponding lighthouse ticket is [944](https://play.lighthouseapp.com/projects/82401-play-20/tickets/944-scalatemplatecompiler-fails-to-lookup-scala-libraryjar-on-windows-xp).

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