Sweep the last few uses of legacy AstNode.getAFlowNode() in tests over to
explicit ControlFlowNode joins after the shared-CFG migration. importflow.ql
needs the new Cfg::ControlFlowNode/CompareNode types because DataFlow::Node.
asCfgNode() now returns the shared-CFG node.
Also extend ImportResolution::allowedEssaImportStep to walk back through
uncertain-write SSA inputs, so that a later 'from X import *' does not hide
the preceding explicit (re)assignment from module-export resolution. Without
this, a reassigned name that survives a wildcard import was no longer
recognised as the module export. Rebless ModuleExport.expected to drop the
legacy 'ControlFlowNode for' toString prefix and pick up the two correct rows
exposed by the fix.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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