This PR addresses the performance issue described in #1409.

#1412 has partially improved performance by memoizing frequently reused ignore patterns.
These commits now improve performance on my Windows 10 machine further by:

• Use Java NIO Files.walk to efficiently walk the build context directory. It visits files and subdirectories in parallel and filters any items efficiently ignored
• Use java.nio.file.Path#toUri instead of java.io.File#toUri, because it executes faster

With #1412 and these code changes, build context scans in my unit test setup (see #1412) on my Windows 10 machine now pass in about 6 seconds, while they previously used to take around 27 seconds.