M0005 imported and not used

example $ marmot build .
error[M0005]: "os" imported and not used
 --> ./main.go:3:8
  |
3 | import "os" // nothing uses os
  |        ^^^^ imported here but not used
  |
  = help: if it is imported only for its side effects, write `import _ "os"`

error: build failed with 1 error
for more about an error, run `marmot explain M0005`

package main

import "os" // nothing uses os

func main() {}