Package importer provides access to export data importers.
For returns an Importer for importing from installed packages for the compilers "gc" and "gccgo", or for importing directly from the source if the compiler argument is "source". In this latter case, importing may fail under circumstances where the exported API is not entirely defined in pure Go source code (if the package API depends on cgo-defined entities, the type checker won't have access to those).
If lookup is nil, the default package lookup mechanism for the given compiler is used.
BUG(issue13847): For does not support non-nil lookup functions.
type Lookup func(path string) (io.ReadCloser, error)
A Lookup function returns a reader to access package data for a given import path, or an error if no matching package is found.