Fix the --mirror option to correctly handle modules with multiple licenses (#475)
Fix the --mirror option to correctly handle modules with multiple licenses (#475)

The previous approach would put all the licenses at the root of a directory
with the module's name. This ended up overwriting license files when a given
module has multiple license files with the same name in different subdirectories.
The subdirectory structure is now preserved in the generated licenses directory
in order to avoid overwrites.
istio/tools,istio/tools,istio/tools,istio/tools,istio/tools,istio/tools