cmd/go: warn if 'go install' is called on a non-main package pattern in module mode

[bcmills]

This issue is a companion to proposal #41696, brought to my attention via https://stackoverflow.com/q/64462190.

In module mode, non-main packages lack an install target, and go install for such packages is equivalent to go build (compare #46911).

As such, a call to go install with a pattern containing only non-main packages is at best confusing: at best, the user has conflated go install with go build, and at worst, they are trying to install a binary but have named a corresponding library instead.

The latter case crops up surprisingly frequently. For example, the library golang.org/x/tools/imports is easily confused with the executable golang.org/x/tools/cmd/goimports, and the library github.com/gobuffalo/buffalo is easily confused with the executable github.com/gobuffalo/buffalo/buffalo.

I don't think we need to actually remove the ability to call go install on a non-main package, but we should at least log something to clear up the user's likely confusion.

CC @jayconrod @matloob

[ianlancetaylor]

@bcmills This is in the 1.18 milestone; time to move to 1.19? Thanks.