Fix issues using DummyGameLocator with libraries

[colinmcdonald22]

Issue #1: DummyGameLocator#gameLibraries() returns a closed Stream

As Stream is AutoCloseable, the stream returned by DummyGameLocator's gameLibraries() method was getting closed immediately after the try-catch block. This resulted in the following error:

[11:00:03] [main/INFO]: Preparing the game...
Exception in thread "main" java.lang.IllegalStateException
	at java.base/java.nio.file.FileTreeIterator.hasNext(FileTreeIterator.java:102)
	at java.base/java.util.Iterator.forEachRemaining(Iterator.java:132)
	at java.base/java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1939)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
	at space.vectrix.ignite.IgniteBootstrap.run(IgniteBootstrap.java:157)
	at space.vectrix.ignite.IgniteBootstrap.main(IgniteBootstrap.java:74)

This IllegalStateException is being thrown here: https://github.com/JetBrains/jdk8u_jdk/blob/master/src/share/classes/java/nio/file/FileTreeIterator.java#L102-L103, as the stream is already closed.

Issue #2: Incorrect .jar extension check

.endsWith() on Path has some interesting semantics, and doesn't just match a suffix on the file name like we're expecting it to. Changing to .toString().endsWith() fixes this.