This package imports all "importable" packages, i.e., packages that:

- are not applications ("main")
- are not internal
- and that have non-test go-files

We do this to verify that our code can be consumed as a dependency
in "module mode". When using a dependency that does not have a go.mod
(i.e.; is not a "module"), go implicitly generates a go.mod. Lacking
information from the dependency itself, it assumes "go1.16" language
(see [DefaultGoModVersion]). Starting with Go1.21, go downgrades the
language version used for such dependencies, which means that any
language feature used that is not supported by go1.16 results in a
compile error;

    # github.com/docker/cli/cli/context/store
    /go/pkg/mod/github.com/docker/cli@v25.0.0-beta.2+incompatible/cli/context/store/storeconfig.go:6:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    /go/pkg/mod/github.com/docker/cli@v25.0.0-beta.2+incompatible/cli/context/store/store.go:74:12: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)

These errors do NOT occur when using GOPATH mode, nor do they occur
when using "pseudo module mode" (the "-mod=mod -modfile=vendor.mod"
approach used in this repository).

As a workaround for this situation, we must include "//go:build" comments
in any file that uses newer go-language features (such as the "any" type
or the "min()", "max()" builtins).

From the go toolchain docs (https://go.dev/doc/toolchain):

> The go line for each module sets the language version the compiler enforces
> when compiling packages in that module. The language version can be changed
> on a per-file basis by using a build constraint.
>
> For example, a module containing code that uses the Go 1.21 language version
> should have a go.mod file with a go line such as go 1.21 or go 1.21.3.
> If a specific source file should be compiled only when using a newer Go
> toolchain, adding //go:build go1.22 to that source file both ensures that
> only Go 1.22 and newer toolchains will compile the file and also changes
> the language version in that file to Go 1.22.

This file is a generated module that imports all packages provided in
the repository, which replicates an external consumer using our code
as a dependency in go-module mode, and verifies all files in those
packages have the correct "//go:build <go language version>" set.

To test this package:

    make -C ./internal/gocompat/
    GO111MODULE=off go generate .
    go mod tidy
    go test -v
    # github.com/docker/docker/libnetwork/options
    ../../libnetwork/options/options.go:45:25: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    # github.com/docker/docker/libnetwork/internal/setmatrix
    ../../libnetwork/internal/setmatrix/setmatrix.go:13:16: type parameter requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:13:18: predeclared comparable requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:14:20: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:20:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:31:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:43:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:59:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:80:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:93:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:104:10: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/internal/setmatrix/setmatrix.go:104:10: too many errors
    # github.com/docker/docker/libnetwork/config
    ../../libnetwork/config/config.go:35:47: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/config/config.go:47:41: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/config/config.go:63:55: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../libnetwork/config/config.go:95:63: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    # github.com/docker/docker/testutil
    ../../testutil/helpers.go:80:9: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
    # github.com/docker/docker/builder/builder-next/adapters/containerimage
    ../../builder/builder-next/adapters/containerimage/pull.go:72:4: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    ../../builder/builder-next/adapters/containerimage/pull.go:200:19: type instantiation requires go1.18 or later (-lang was set to go1.16; check go.mod)
    FAIL	gocompat [build failed]
    make: *** [Makefile:5: verify] Error 1

[DefaultGoModVersion]: https://github.com/golang/go/blob/58c28ba286dd0e98fe4cca80f5d64bbcb824a685/src/cmd/go/internal/gover/version.go#L15-L24
[2]: https://go.dev/doc/toolchain

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>