[IDEABKL-7100] Create an instance of PsiElementFinderImpl via the extension point 'com.intellij.java.elementFinder'

Probably an instance variable com.intellij.psi.impl.JavaPsiFacadeImpl#myElementFinders and a method com.intellij.psi.impl.JavaPsiFacadeImpl#calcFinders could be removed.
And a method com.intellij.psi.impl.JavaPsiFacadeImpl#finders could just delegate to myProject.getExtensions(PsiElementFinder.EP_NAME).