Clean up BUILD rules

* Use "_" instead of "-" in target names to match the more common
  naming style, except the plugin name find-owners.
* Use shard_count=4 to speed up findowners_IT_tests.
* Use FIND_OWNERS_*_{SRCS,DEPS} to factor out lists
  of source and dependent files.
* Inline the use of PLUGIN_DEPS and MODULE.
* No need of tags.

Change-Id: I50d348191b5910c6f3343bdd0229b1e381e8741e