Package internalsrv provides a simple middleware that (a) prevents access to internal locations and (b) allows to return files from internal location by setting a special header, e.g. in a proxy response.
The package is named internalsrv so as not to conflict with Go tooling convention which treats folders called "internal" differently.
Internal middleware protects internal locations from external requests - but allows access from the inside by using a special HTTP header.
ServeHTTP implements the httpserver.Handler interface.