Using the $ variable (recommended)
From the package text/template documentation:
When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.
As @Sandy points out, it is therefore possible to access the Path in the outer scope using $.Path.
const page = `{{range .Files}}<script src="https://stackoverflow.com/questions/17284222/{{html $.Path}}/js/{{html .}}"></script>{{end}}`
Using a custom variable (old answer)
Found one answer just minutes after posting.
By using a variable, a value can be passed into the range scope:
const page = `{{$p := .Path}}{{range .Files}}<script src="https://stackoverflow.com/questions/17284222/{{html $p}}/js/{{html .}}"></script>{{end}}`