Error in importing custom packages in Go Lang

to export a function into another package the function identifier must start with a capital letter.

you would need to make your function exportable with an uppercase for its name:

func Fastget(...

Used as:

n:=libfastget.Fastget(url,4,filename)

The spec mentions: "Exported identifiers":

An identifier may be exported to permit access to it from another package. An identifier is exported if both:

• the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
• the identifier is declared in the package block or it is a field name or method name.

All other identifiers are not exported.