Build fails with new caddy since switching to go modules

[abom]

When did you download File Browser from caddyserver.com?
Building from source

What is your entire Caddyfile?
None

Description
Trying to build caddy with filebrowser from source, but the build fails since caddy switched to go modules.

Detailed issue with errors can be found here.

Expected behaviour
Successful build.

What is happening instead?
Mentioned at caddyserver/caddy#2549

How to reproduce?
Mentioned at caddyserver/caddy#2549

[pepa65]

Same, go 1.12 here; this is the file that I'm trying to build:

package main
import (
  "github.com/mholt/caddy/caddy/caddymain"
    _ "github.com/filebrowser/caddy"
)

func main() {
  caddymain.Run()
}

go build outputs:

go: finding github.com/mholt/caddy/caddy/caddymain latest
go: finding github.com/mholt/caddy/caddy latest
go: finding github.com/GeertJohan/go.rice/embedded latest
go: finding github.com/filebrowser/filebrowser/errors latest
go: finding github.com/filebrowser/filebrowser/settings latest
go: finding github.com/filebrowser/filebrowser/users latest
go: finding github.com/filebrowser/filebrowser/http latest
go: finding github.com/filebrowser/filebrowser/auth latest
go: finding github.com/filebrowser/filebrowser/storage latest
go: finding github.com/filebrowser/filebrowser/storage/bolt latest
build build: cannot load github.com/filebrowser/filebrowser/auth: cannot find module providing package github.com/filebrowser/filebrowser/auth

[eine]

@hacdias, any guess?