[SPARK-3788] [yarn] Fix compareFs to do the right thing for HDFS namespaces.

HA and viewfs use namespaces instead of host names, so you can't
resolve them since that will fail. So be smarter to avoid doing
unnecessary work.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #2649 from vanzin/SPARK-3788 and squashes the following commits:

fedbc73 [Marcelo Vanzin] Update comment.
c938845 [Marcelo Vanzin] Use Objects.equal() to avoid issues with ==.
9f7b571 [Marcelo Vanzin] [SPARK-3788] [yarn] Fix compareFs to do the right thing for HA, federation.
[SPARK-3848] yarn alpha doesn't build on master

yarn alpha build was broken by #2432
as it added an argument to YarnAllocator but not to yarn/alpha YarnAllocationHandler
commit https://github.com/apache/spark/commit/79e45c9323455a51f25ed9acd0edd8682b4bbb88

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2715 from sarutak/SPARK-3848 and squashes the following commits:

bafb8d1 [Kousuke Saruta] Fixed parameters for the default constructor of alpha/YarnAllocatorHandler.