Support vendoring Go projects with a committed go.work that assumes workspace mode

[jfkw]

Add support to obs-service-go_modules for vendoring with upstream projects using workspaces. When a Go project has a workspace go.work and go.work.sum committed to git, go mod vendor will fail with error:

go mod vendor
go: 'go mod vendor' cannot be run in workspace mode.
Run 'go work vendor' to vendor the workspace or set 'GOWORK=off' to exit workspace mode.

When following the recommendation GOWORK=off, the go.sum committed to git may be incomplete otherwise out of sync if developers are routinely using only the workspace when building:

go-tpm-tools> GOWORK=off go mod vendor
go: github.com/google/go-tpm-tools/internal/util imports
        cloud.google.com/go/compute/metadata: 
        missing go.sum entry for module providing package cloud.google.com/go/compute/metadata
       (imported by github.com/google/go-tpm-tools/internal/util); to add:
        go get github.com/google/go-tpm-tools/internal/util
go: github.com/google/go-tpm-tools/internal/util imports
        github.com/golang-jwt/jwt/v4:
        no required module provides package github.com/golang-jwt/jwt/v4; to add it:
        go get github.com/golang-jwt/jwt/v4
go: github.com/google/go-tpm-tools/internal/util imports
        golang.org/x/net/http2:
        missing go.sum entry for module providing package golang.org/x/net/http2
        (imported by github.com/google/go-tpm-tools/internal/util); to add:
        go get github.com/google/go-tpm-tools/internal/util
...

Research additional upstream projects with committed workspaces and document whether a standard sequence of go mod operations that can be used to reliably vendor such projects. If no single standard sequence is possible, logic can be built into the service implementation to detect and do the right thing for known layouts as encountered.

[jfkw]

To get the upstream project github.com/google/go-tpm-tools to work with go mod vendor in non workspace mode, first run go mod tidy.

[jfkw]

Refs #47

[johanneskastl]

Any news on this one?