fixed ExplodedArchive which fail on windows when run exploded jar file

When run exploded jar on windows `java org.springframework.boot.loader.JarLauncher`

You will get
```
java.lang.IllegalStateException: No 'Start-Class' manifest entry specified in file:C:\foo\bar/
        at org.springframework.boot.loader.archive.Archive.getMainClass(Archive.java:56)
        at org.springframework.boot.loader.ExecutableArchiveLauncher.getMainClass(ExecutableArchiveLauncher.java:73)
        at org.springframework.boot.loader.Launcher.launch(Launcher.java:53)
        at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:42)
```

The fix will use platform independent path from `file.toURI().getPath()` instead `file.getAbsolutePath()`.

After the fix, `mvn clean install` on spring-boot project root pass. And it pass on windows as well for command `mvn clean install -f spring-boot-tools/spring-boot-loader/pom.xml`