Fix calculation of base package

[fabapp2]

What needs to be done

Calculating base packages should return the common parent package, even if it is the default (no) package.
In BasePackageCalculator.calculateBasePackage(List<JavaSource> javaSources)

Why it needs to be done

For e.g. two classes com.acme.some.A and com.acme.other.B the current calculation does fail as it does not recognize com.acme as possible common base package.

Acceptance Criteria

Given base package should be calculated for an application
When two classes com.acme.some.A and com.acme.other.B exist
Then the returned base package should be com.acme

Given base package should be calculated for an application
When two classes com.acme.some.A and com.example.other.B exist
Then the returned base package should be com

Given base package should be calculated for an application
When two classes com.acme.some.A and org.example.other.B exist
Then the returned base package should be ""

See JavaSourceSetImplTest.getBasePackageShouldReturnDefaultIfNoDistinctRootPackageExists()
This test should expect com.vmware.sbm instead of the default base package.

Additional Information

All recipes/actions calculating the base package for Spring components and configurations should check if the returned base package is the default package/empty and handle this situation. At least initialize-spring-boot-migration would be affected.
For Spring applications to work without one common base package, a @ComponentScan could be used.

[soumyaPrakashB]

I have raised a PR for this issue: #706
Please review.