generate --output-multiple-files panics on a template with no file

[crhntr]

muxt generate --output-multiple-files panics when a template has no file of its own, for example one added with template.New("GET /health Health()").Parse(...).

panic: runtime error: index out of range [0] with length 0

github.com/typelate/muxt/internal/generate.accumulateReceiverMethods.func1(...)
slices.IndexFunc[...](...)
github.com/typelate/muxt/internal/generate.accumulateReceiverMethods(...)
github.com/typelate/muxt/internal/generate.hydrateGroup(...)
github.com/typelate/muxt/internal/generate.TemplateRoutesFiles(...)

accumulateReceiverMethods reads field.Names[0].Name for every field in the receiver interface. With --output-multiple-files the main file's interface embeds the per-file interfaces, and an embedded field has no Names, so the lookup panics as soon as a route in the main file adds a method.

Reproduced with a binary built from f7e9368:

module server, index.gohtml holding {{define "GET /{$} Home()"}}, and

var templates = template.Must(template.Must(template.ParseFS(templateFiles, "*.gohtml")).
	New("GET /health Health()").Parse(`ok`))

muxt generate --use-receiver-type=T --output-multiple-files

Found while writing snapshot tests for --output-multiple-files; the archive in #151 documents only what does work, because this case panics.