86732509 updated the plugin so that the application plugin is no longer
applied by default. This exposed three problems:

 1. bootRepackage may run before findMainClass has run, leaving it with
    an unknown main class.
 2. findMainClass may run before the classes have been built, making it
    unable to find the main class by examining the class files
 3. The project's mainClassName property was still being used as a
    convention for the bootRun task's main property. If the application
    plugin has not be applied, then this property does not exist.

The first problem has been addressed by configuring bootRepackage to
depend on findMainClass.

The second problem has been addressed by configuring the main source
set's output as an input of findMainClass, and configuring findMainClass
to depend on the tasks that build the output.

The third problem has been addressed by only using the mainClassName
property if it exists and its value is not null. We then fallback to
using the mainClassName property on the project's extra properties in
the same way. 

See gh-2679