LSP: Auto import of types in code completion fixed.

[dbalek]

SourceUtils.resolveImport(...) used to prevent clashes in import statements introduced by code completion.
However, using of SourceUtils.resolveImport(...) has a side-effect: in addition to rewrite(oldTree, newTree) modifications, it also modifies importScopes data structures (in order to work reliably if called repeatedly within a single task). Usually in NB, every ModificationTask is followed by commit() on ModificationResult produced. This call changes the underlying document which invalidates the corresponding Source, so it has to re-parsed for using in the next task (instead of using the cached parser result with corrupted scopes). However in NBLS, only diffs are taken from the ModificationResult and sent to client side for handling (without calling commit()). That is why the corresponding Source has to be invalidated explicitly.