licenses and compliance with apache 2.0

[sreev]

epic for all about licenses used in heron.

heron on apache 2.0 license
third party libraries used on apache 2.0 or compatible licenses
todo's when not compatible
packaging for releases

[sreev]

in an attempt to simplify the 'include licenses' in heron packaging:
there seems an easier way to check all licenses used in heron.
$ bazel license

there is an option --check_licenses that can be used in the build process.
https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/runtime/commands/LicenseCommand.java

can it be extended for python, cpp and scala libraries ?
in pure java world maven plugin exists.
https://www.mojohaus.org/license-maven-plugin/index.html

bazelbuild/bazel#7444
some insights into license checks in bazel being implemented...

are BUILD files excluded from license header ?

This is how other projects dealing with licenses.

1. Make a profile such that the jar is not included in the release /
   optional https://issues.apache.org/jira/browse/RYA-373 (Apache Rya)
2. Replace the GPL w/ Classpath Exception with something Apache licensed.
   https://issues.apache.org/jira/browse/LEGAL-396 (Apache Lucene)
3. It was also suggested to declare jmh in provided scope so we don't ship
   it. (not sure if this is possible. Does JDK runtime provide jmh?)
   https://issues.apache.org/jira/browse/LEGAL-399 Apache Calcite