Report debug diagnostics in file packager if EMCC_DEBUG=1. Make file pac...

...kager skip all path components that have a path starting with '.', like ('.git/xxx/yyy/'), or the filename starts with '.', or the file is a Win32 hidden file. Explicitly specified files on the command line are always packaged, even if they start with '.' or are hidden. Add unit tests to check that it works.

Ran unit tests to check that browser.test_preload_file works.