$ go doc comment.Parser
[...]
        // Setting LookupPackage to nil is equivalent to setting it to
        // a function that always returns "", false.
        LookupPackage func(name string) (importPath string, ok bool)
[...]

$ go doc comment.DefaultLookupPackage
package comment // import "go/doc/comment"

func DefaultLookupPackage(name string) (importPath string, ok bool)
    DefaultLookupPackage is the default package lookup function, used when
    Parser.LookupPackage is nil. It recognizes names of the packages from the
    standard library with single-element import paths, such as math, which would
    otherwise be impossible to name.
[...]