`fileglob.Glob` doesn't work with an absolute path on Windows

[rhysd]

Related to #19. I saw this comment. So I used filepath.ToSlash but it still doesn't work in this case.

Repro

Run the following code on Windows:

import (
  "fmt"
  "path/filepath"
  "github.com/goreleaser/fileglob"
)

func main() {
  x, err := fileglob.Glob(filepath.ToSlash(`C:\Windows`))
  if err != nil {
    panic(err)
  }
  fmt.Println(x)
}

Expected behavior

Since C:\Windows exists, it should print a slice with one element.

Actual behavior

It panicked with the following error:

stat static prefix ./C:/Windows: stat C:/Windows: invalid argument

filepath.ToSlash(`C:\Windows`) returns "C:/Windows". The function didn't seem to consider the C: drive letter and misunderstood that the path was relative.