[test] Rename all module.map files to module.modulemap

`module.map` as a module map name has been discouraged since 2014, and
Clang will soon warn on its usage. This patch renames all instances of
`module.map` in the Swift tests to `module.modulemap` in preparation
for this change to Clang.

rdar://106123303

test/SourceKit/Mixed/Inputs/Mixed.framework/Modules/module.modulemap

@@ -0,0 +1,4 @@
+framework module Mixed {
+  header "Mixed.h"
+  export *
+}