Missing go.mod in caddy submodule

[JaimeValdemoros]

This is a pretty minor issue so please feel free to close it if you don't think it's worth your time.

Context

The nix/nixpkgs build system provides an integration to build caddy with a list of extensions (using xcaddy or a process like xcaddy under the covers). This lets me write something like the following and get a reproducible build build of caddy with go-pmtiles in just a couple of lines:

# default.nix

{ caddy }:
caddy.withPlugins {
    plugins = [
        "github.com/protomaps/go-pmtiles/caddy@v1.28.0"
    ];
    hash = "sha256-QVyV1PPYqQQnS531BDzp8Equ0le1AzyOHbsCjeOVlew=";
}

Issue

As part of its checks, this integration looks at caddy build-info and checks that all of the requested plugins show up. Unfortunately this check currently isn't working for go-pmtiles because of a mismatch between the requested plugin and the output in build-info (example using xcaddy):

$ xcaddy build --with github.com/protomaps/go-pmtiles/caddy@v1.28.0
$ ./caddy/caddy build-info | grep pmtiles
dep     github.com/protomaps/go-pmtiles v1.28.0 h1:Q23U0DqfVE0PM2ZVcehgZ4yIjd73jcK5UMKcX6K5R3c=

(notice github.com/protomaps/go-pmtiles/caddy vs. github.com/protomaps/go-pmtiles).

I raised NixOS/nixpkgs#430090 as a result to look into it.

Request

@stepbrobd has pointed out this is likely due to a missing go.{mod,sum} under the caddy directory. By way of comparison, building with github.com/darkweak/storages/badger/caddy does show up the full path in caddy build-info:

$ ./result/bin/caddy build-info | grep badger
dep     github.com/darkweak/storages/badger     v0.0.15
dep     github.com/darkweak/storages/badger/caddy       v0.0.15
dep     github.com/dgraph-io/badger/v3  v3.2103.5
build   -tags=nobadger,nomysql,nopgx

This is a link to badger's go.mod: https://github.com/darkweak/storages/blob/main/badger/caddy/go.mod

Would it be feasible to get a go.mod added to the directory so it acts like a proper module? I'm not hugely familiar with go tooling, so I don't know what the impact would be.

[bdon]

I attempted to add a go.mod in the caddy subdirectory like this:

go mod init github.com/protomaps/go-pmtiles/caddy

But then go mod tidy fails with this:

go: github.com/protomaps/go-pmtiles/caddy: ambiguous import: found package github.com/protomaps/go-pmtiles/caddy in multiple modules:
        github.com/protomaps/go-pmtiles v1.28.0 (/Users/.../go/pkg/mod/github.com/protomaps/go-pmtiles@v1.28.0/caddy)
        github.com/protomaps/go-pmtiles/caddy (/Users/.../protomaps/go-pmtiles/caddy)

I'm not an expert on Caddy builds, go packaging or Nix so if you can design a working PR to have a go.mod in the caddy dir, that does not have other maintainability side effects (like affecting the root go.mod), I'm open to merging that.