cmd/go: suggest 'go get' for packages in the main module instead of missing external package paths

[bcmills]

In #45979 (comment), I noted that go get in a go 1.17 module adds its arguments as // indirect.

As an intentional consequence of #45965, we place the // indirect dependencies in a separate section of the go.mod file from the direct dependencies. That potentially makes these new actually-direct dependencies more difficult to spot.

If we saw the import of the missing package from within the main module to begin with, we would add it without the erroneous // indirect marking, and avoid the churn of moving it between sections. So I think we should suggest a command that does that, instead of suggesting go get on the specific missing package(s).

That would also help to eliminate some of the redundancy in the go get hints (#43653).

CC @jayconrod @matloob @stevetraut

[bcmills]

This is closely related to #46710.

[bcmills]

This would also avoid the case where we suggest go get on a package path, but the resulting go get command adds a module dependency that does not contain a package at its root (as in the example reported in #57473).

[bcmills]

(CC @matloob @samthanawalla)