build: Namespace installed header files
build: Namespace installed header files

This installs the public header files in an eos-shard-0 subdirectory, so
that a subsequent eos-shard-1 would be parallel installable.

[endlessm/eos-sdk#3150]

endlessm/eos-shard,endlessm/eos-shard,endlessm/eos-shard