strip import comments

```
Go 1.4 allows packages to specify a "canonical" import
path with a comment of the form
    package foo // import "bar/foo"
If the package is imported from a path other than
"bar/foo", the go tool will reject it. We circumvent
this check by removing such comments, so that
dependencies can be imported from their rewritten import
paths inside Godeps/_workspace.
```

See also http://golang.org/s/go14customimport.
You could make this cheaper and run faster by assuming that the source file is at least vaguely gofmt'd.

var pkgPrefix = []byte("package ") // up higher

if !bytes.HasPrefix(line, pkgPrefix) { // fast path; this will skip all but one line in the file
  return line
}
// ... do the regexp