WorkManager.isLockAlreadyAcquired() is not a public API but it was there
since ever (2003, see e9154ab).

With 28d4794 the method was made package protected, which is technically
OK but unnecessarily breaks all clients that used non-API code before
(Xtext is one example:
org.eclipse.xtext.ui.workspace.WorkspaceLockAccess.isWorkspaceLockedByCurrentThread(IWorkspace)).

To avoid unnecessarily breaking existing code, this method should be
made public again.

This shouldn't be an issue, as we had no issues having this code public
for 20 years, should be OK to keep it public for the next 20 years.

Fixes https://github.com/eclipse-platform/eclipse.platform/issues/490