GRAILS-10502 - improve plugin service alias handling

We only create the plugin service alias if a bean does not already exist with the potential alias name.  This change extends that to also make sure that the potential alias name does not conflict with an existing alias.

Relevant functional tests: https://github.com/grails/grails-functional-tests/commit/d112ee03475254c43956d672f705abed4d34250c
Merge branch '2.3.x'