xds: move code to a root level xds directory

[easwars]

Move code out of balancer/xds.

We will have a root level xds/ directory which will eventually contain
all xDS implementation including balancer, resolver, client etc.

For now, we will have an xds/experimental/ directory where all the code
will live, and an xds/internal/ directory for stuff internal to the xds
package. Once this is deemed stable enough, it will be moved to the
top-level xds/ directory.

Also, moved balancer/internal/wrr to xds/internal/wrr as this code is
only used by xds today.

[easwars]

@menghanl Changed the directory structure as per our offline discussion. PTAL.

[easwars]

@menghanl
Moved the balancer/internal/wrr package to internal/wrr as discussed offline.