Go Buildpack unable to find a local module. What am I missing?

I am trying to build and launch a Go app on https://fly.io but when it comes to building it fails to find my test and templates package seen below:

    .
    ├── cmd
    │   ├── doc
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── git
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── imp
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── log
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── met
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── orc
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── pub
    │   │   ├── go.mod
    │   │   └── main.go
    │   ├── rep
    │   │   ├── go.mod
    │   │   └── main.go
    │   └── web
    │       ├── fly.toml
    │       ├── go.gen
    │       ├── go.mod
    │       ├── go.sum
    │       ├── handlers.go
    │       ├── handlers_test.go
    │       ├── main.go
    │       ├── main_test.go
    │       ├── router.go
    │       └── router_test.go
    ├── CONTRIBUTING.md
    ├── go.mod
    ├── go.work
    ├── internal
    ├── LICENSE.txt
    ├── main.go
    ├── pctl
    ├── pkg
    │   ├── **templates**
    │   │   ├── base.qtpl
    │   │   ├── base.qtpl.go
    │   │   ├── go.mod
    │   │   ├── go.sum
    │   │   ├── whoami.qtpl
    │   │   └── whoami.qtpl.go
    │   └── **test**
    │       ├── go.mod
    │       └── test.go
    └── README.md

https://paketo.io/docs/reference/go-reference/#package-management-with-go-modules states:
> The buildpack will vendor dependencies using go modules if the app source code contains a go.mod file. During the build phase, the go-mod-vendor buildpack(opens in a new tab) checks to see if the application requires any external modules and if it does, runs the go mod vendor command for your app. The resulting vendor directory will exist in the app’s root directory and will contain all packages required for the build.

Looking at the build log, I see that go mod vendor was indeed run.
> Paketo Buildpack for Go Distribution 2.2.3
> Resolving Go version
> Candidate version sources (in priority order):
> go.mod -> ">= 1.19"
> <unknown> -> ""
>
> Selected Go version (using go.mod): 1.19.5
>
> Executing build process
> Installing Go 1.19.5
> Completed in 35.526s
>
> Generating SBOM for /layers/paketo-buildpacks_go-dist/go
> Completed in 0s
>
> Paketo Buildpack for Go Mod Vendor 1.0.7
> Checking module graph
> Running 'go mod graph'
> Completed in 1.166s
>
> Executing build process
> Running 'go mod vendor'
> Completed in 9.851s
>
> Generating SBOM for /workspace/go.mod
> Completed in 21ms
>
> Paketo Buildpack for Go Build 2.0.8
> Executing build process
> Running 'go build -o /layers/paketo-buildpacks_go-build/targets/bin -buildmode pie -trimpath .'
> Failed after 611ms
> failed to execute 'go build': exit status 1
> handlers.go:5:2: cannot find package "." in:
> /workspace/vendor/templates
> main.go:8:2: cannot find package "." in:
> /workspace/vendor/test
> ERROR: failed to build: exit status 1
> Error failed to fetch an image or build from source: executing lifecycle: failed with status code: 51

handlers.go:5:2: "templates"
main.go:8:2: "test"

What I've done:

1. I've changed the module names to a url (i.e. test -> some.com/test) and used a replace directive in go.mod to point to the modules.
2. I've ran go mod vendor and seen what gets generated... some.com is there!
3. I've tried building both locally and remotely with fly launch. Both result in no such file.

I am trying a private repo now.

Not a great solution but one nonetheless is to edit your host file to have your module pointed to a local file server.

# vim /etc/hosts
127.0.0.1    <module name>

I'll edit this answer if I ever find out if the problem lies with fly or Buildpack. The problem being when fly runs go mod vendor it doesn't respect the replace directive in go.mod.

go mod vendor ran on it's own will copy everything correctly as long as the module's name is in url format. It doesn't have to be a valid url.

The other solution is to simply host your code at a valid url and be done with it. There might be a flag I'm missing but I have not found it yet. I'm hoping anyone that runs into this might find this answer helpful in the meantime.