This branch is a proof-of-concept, re-organizing the client structure to be more clear, and adding aliases to protect existing functionality:

• the legacy/classic client was moved into google.cloud.bigtable.classic
  • imports referencing google.cloud.bigtable remain working as an alias
• the admin client was moved into google.cloud.bigtable.admin
  • google.cloud.bigtable_admin_v2 remains as an alias
• internal references were updated to point to new canonical locations

If we go through with this change:

• add tests to import packages from both locations in different ways, and make sure all imports continue working (ex: import google.cloud.bigtable.Client, from google.cloud.bigtable import Client, etc)
• update owlbot.py to move gapic generated admin files to new location