show where element is exported from

[CelineMaterna]

when entries are exported it would be nice to see from where they were exported.
Exported classes come from the export XYZ or export * in a module. It basically says that one also gets entries from other modules when importing this one.
If you look at https://github.com/toitware/toit/blob/master/lib/core/core.toit you can see that our core library is, in fact, just reexporting entries and doesn't have any members on its own.