Run `semantic3` on imported modules, and emit new symbols with `available_externally` linkage. This allows the inliner to inline functions from other modules while telling the code generator to ignore those functions (treat them as declarations) Still generates a few extra `TypeInfo`s and strings... Disabled when generating debug info because I don't really understand it, and it doesn't like this.

[Environment]
DFLAGS=-I@RUNTIME_DIR@/import -L-L%@P%/../lib -defaultlib=@RUNTIME_AIO@ -debuglib=@RUNTIME_AIO@