In Go, how to import function directly, without need to prefix with the package name when I call it?

I guess this doesn't really answer your question, but if you want, you can actually call the methods without explicitly stating the package - just import with a . in front of the names (but this is not recommended; see below):

package main

import (
  . "fmt"
  . "io/ioutil"
)

func main () {
  content, err := ReadFile("testfile")
  if err != nil {
    Println("Errors")
  }
  Println("My file:\n", string(content))
}

Note @jimt's comment below - this practice is not advised outside of tests as it could cause name conflicts with future releases. Also, definitely agree with @DavidGrayson's point of being nicer to read/see where things come from.

you can import and rename the package name, eg:

    import (  
        .     "fmt"       // no name, import in scope  
        File  "io/ioutil" // rename ioutil to File
        _     "net"       // net will not be available, but init() inside net package will be executed
    )

See also https://golang.org/ref/spec#Import_declarations

I can't really speak for the designers of the Go language, but it is nice to be able to quickly tell where the method you are calling is defined. It is also nice to see a list of all the packages your are using at the top of the file. This is not redundant.

As you said, Java requires you to say Files.readAllLines and similarly go requires you to write ioutil.ReadFile.