fix: build with go 1.23 by tracking modules.txt

[obreitwi]

Fixes: #117

Since go 1.23, vendor/modules.txt is required.

Hence, this is a first attempt to track modules.txt via gomod2nix.

Since, unfortunately, module.txt employs a bespoke custom format in which
varying numbers of # hold meaning I deemed it more stable to not reverse engineer it.

Instead, we generate it when running gomod2nix by executing go mod vendor and tracking its content in gomod2nix.toml.

Not the most elegant solution, but it works and enables us to use gomod2nix with go v1.23.

Any comments appreached…

[hannesg]

I tested it locally and it indeed works with go 1.23.

[hannesg]

I checked why the unit test for the cli-args fails and the situation is somewhat complicated.

1. go build -mod=vendor is now checking that imports are in vendor/modules.txt.
2. The import that is missing in the build ( github.com/yuin/goldmark ) is in the go.mod of golang.org/x/tools but not in the vendor/modules.txt we generate.
3. It's not in our vendor/modules.txt because the test only builds a subpackage that doesn't use it. Go can detect this because we create our own temporary module and only reference the subpackage there. The resulting go.mod is

module gomod2nix/dummy/package

go 1.23.1

require golang.org/x/tools v0.1.11

require (
        golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 // indirect
        golang.org/x/sys v0.0.0-20211019181941-9d821ace8654 // indirect
)

and so the resulting vendor/modules.txt is

# golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4
## explicit; go 1.17
golang.org/x/mod/semver
# golang.org/x/sys v0.0.0-20211019181941-9d821ace8654
## explicit; go 1.17
golang.org/x/sys/execabs
# golang.org/x/tools v0.1.11
## explicit; go 1.17
golang.org/x/tools/cmd/stringer
golang.org/x/tools/go/gcexportdata
golang.org/x/tools/go/internal/gcimporter
golang.org/x/tools/go/internal/packagesdriver
golang.org/x/tools/go/packages
golang.org/x/tools/internal/event
golang.org/x/tools/internal/event/core
golang.org/x/tools/internal/event/keys
golang.org/x/tools/internal/event/label
golang.org/x/tools/internal/gocommand
golang.org/x/tools/internal/packagesinternal
golang.org/x/tools/internal/typeparams
golang.org/x/tools/internal/typesinternal

This is absolutely correct. If you run go build -mod=vendor golang.org/x/tools/cmd/stringer on that, it works.

4. So why does go then later claim that it's missing? That's because we are not building our temporary go module but from the source of golang.org/x/tools which does include the dependency in its go.mod.

So, how to fix this? I think ultimately we would need to bring the environment that generates the gomod2nix.toml and the environment that consumes it closer together which is tedious :(