This commit clarifies the mixed licensing in the elasticsearch
repository. A few points of note:
- we clarify that all code is under the Apache License 2.0 unless noted
otherwise
- we clarify that code under the Elastic License is only in the x-pack
directory
- we clarify that when code is not under the Apache License 2.0 nor the
Elastic License that it is Apache License 2.0 compatible
- we clarify that OSS-builds do not package any code from the x-pack
directory
This commit moves the apache and elastic license files into a new
root level `licenses` directory and rewrites the top level LICENSE.txt
to clarify the repository has a mix of apache and elastic licensed code.