fix(jvm): Let the importer read any path that exists and is not a directory.

[rudo-thomas]

This makes /dev/stdin work again in --tla-code-file, at least on the jvm target. See #448

My #432 broke it.

$ echo '{a:1}' | sjsonnet-0.5.2.jar --tla-code-file f=/dev/stdin -e 'function(f)f'
{
   "a": 1
}

$ echo '{a:1}' | sjsonnet-master-36ecd2a7448d77b729786dd6336029475aba0eae.jar \
  --debug-importer --tla-code-file f=/dev/stdin -e 'function(f)f'
[import /dev/stdin] candidate /dev/stdin
[import /dev/stdin] /dev/stdin is selected as it exists
[read OsPath(/dev/stdin)] file does not exist or is not a file
[read OsPath(/home/r/git/upstream/sjsonnet/<tla-var f>)] file does not exist or is not a file
Exception in thread "main" sjsonnet.Error: Couldn't import file: "/dev/stdin"
	at [Import].(<tla-var f> offset:0)
	at [ValidId f].(<exec>:1:12)

$ echo '{a:1}' | ./out/sjsonnet/jvm/3.3.6/assembly.dest/out.jar --tla-code-file f=/dev/stdin -e 'function(f)f'
{
   "a": 1
}