fix(C4): collectCImportSources recurses into nested namespaces

A named #import c unit declared inside an aliased module sits two
namespace levels deep in the merged tree; the one-level walk (the
extractLibraries/0130 pattern in c_import form) never collected it,
so the unit silently never compiled and its symbols resolved from
whatever process image carried the same names — surfaced by C4's
sqlite migration, where only the version pin could tell the OS copy
from the vendored one.

examples/expected/1623-cimport-unit-in-aliased-module.exit

@@ -0,0 +1 @@
+0

examples/expected/1623-cimport-unit-in-aliased-module.stdout

@@ -0,0 +1 @@
+nested unit answer = 54