…bol graph symbols

rdar://104044227

The snippet symbol graph is currently being generated with the changes in
e013865, however they are not being copied to the correct final
destination.

The `snippet-extract` tool now takes input and output *files* instead of
directories, however there is still one place in the plugin where we are still
treating the output like a directory, so the snippet symbol graph basename path
component looks like a directory, and needs additional basenames to put it in
the right place with the correct file extension.

rdar://104045850

…ponent' into check-snippets