Delete unrelated gradle projects during initialization

[testforstephen]

Signed-off-by: Jinbo Wang [email protected]

Fix #1261

[testforstephen]

In most cases, it's OK to delete those cached gradle projects not in current workspace.

One exceptional case is importing a sub module from a multi module gradle project.
For example, only open the sub module sagan-common from https://github.com/spring-io/sagan to current workspace, the buildship will still import other related modules to jdt too. When reloading the workspace, delete these implicitly referenced gradle projects during initialization didn't cause logic issue, because the following buildship synchronization job will add them back. In order to avoid the unnecessary delete operation, this PR adds logic to bypass this special case.

[fbricon]

thanks @testforstephen