Please consider making the codebase compatible with vendoring

I am currently packaging `google-guest-agent` for SLE/openSUSE and the current build system doesn't allow vendoring on the vanilla source tree. Instead, one has to perform the build first by running `make`, after which `go mod vendor` succeeds. Demonstration: ``` glaubitz@suse-laptop:~/upstream/google-guest-agent> go mod vendor go: github.com/GoogleCloudPlatform/google-guest-agent/cmd/acs_cli imports github.com/GoogleCloudPlatform/google-guest-agent/internal/acp/proto/google_guest_agent/acp: no required module provides package github.com/GoogleCloudPlatform/google-guest-agent/internal/acp/proto/google_guest_agent/acp; to add it: go get github.com/GoogleCloudPlatform/google-guest-agent/internal/acp/proto/google_guest_agent/acp go: github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/agentcrypto imports github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/agentcrypto/proto/credentials: no required module provides package github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/agentcrypto/proto/credentials; to add it: go get github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/agentcrypto/proto/credentials go: github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/snapshot imports github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/snapshot/proto/cloud_vmm: no required module provides package github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/snapshot/proto/cloud_vmm; to add it: go get github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/snapshot/proto/cloud_vmm go: github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/workloadcertrefresh imports github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/workloadcertrefresh/proto/mwlid: no required module provides package github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/workloadcertrefresh/proto/mwlid; to add it: go get github.com/GoogleCloudPlatform/google-guest-agent/cmd/core_plugin/workloadcertrefresh/proto/mwlid glaubitz@suse-laptop:~/upstream/google-guest-agent> make golang build environment: ######################### GOOS=linux VERSION=dev ######################### Making proto dependencies dir... Cloning/updating go-tpm-tools... Already up to date. Making proto dependencies dir... Cloning/updating mwlid-proto... Already up to date. [GEN] cmd/core_plugin/agentcrypto/proto/guest_credentials.proto [GEN] cmd/core_plugin/snapshot/proto/snapshot_service.proto [GEN] internal/acp/proto/agent_controlplane.proto [GEN] cmd/core_plugin/workloadcertrefresh/proto/workload_identity.proto [BIN] cmd/core_plugin/core_plugin [BIN] cmd/ggactl/ggactl_plugin [BIN] cmd/google_authorized_keys/google_authorized_keys [BIN] cmd/google_guest_agent/google_guest_agent [BIN] cmd/google_guest_compat_manager/google_guest_compat_manager [BIN] cmd/gce_metadata_script_runner/gce_metadata_script_runner [BIN] cmd/metadata_script_runner_compat/gce_compat_metadata_script_runner [BIN] cmd/google_authorized_keys_compat/google_authorized_keys_compat glaubitz@suse-laptop:~/upstream/google-guest-agent> go mod vendor glaubitz@suse-laptop:~/upstream/google-guest-agent> ls -l vendor total 68 drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 cel.dev drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 cloud.google.com drwxr-xr-x 19 glaubitz users 4096 Jan 22 09:09 github.com drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 go.bug.st drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 golang.org drwxr-xr-x 6 glaubitz users 4096 Jan 22 09:09 google.golang.org drwxr-xr-x 5 glaubitz users 4096 Jan 22 09:09 go.opentelemetry.io drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 gopkg.in drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 go.uber.org -rw-r--r-- 1 glaubitz users 27534 Jan 22 09:09 modules.txt drwxr-xr-x 3 glaubitz users 4096 Jan 22 09:09 software.sslmate.com glaubitz@suse-laptop:~/upstream/google-guest-agent> ``` Since Linux distributions disable network access during build, running `git clone` during a package build will fail. Additionally, we vendor all the dependencies using `go mod vendor` that the whole build process can take place offline. Thus, it would be very useful if the build system of `google-guest-agent` could be changed to import Go modules the standard way without using `git clone` which would make vendoring the dependencies and building the package much easier and more streamlined for downstream distributions. Thanks for consideration!