warn user when installing an unstable main package

[myitcv]

If the user uses gobin to install a main package that is not part of a module, then by definition the dependencies of that package are "unstable" (probably a better term).

Similarly, if a main package is part of a module, but that module definition is in some way incomplete (e.g. go mod tidy leaves changes in go.{mod,sum}), the results are "unstable".

We could/should warn the user about this somehow/somewhere.

[rogpeppe]

Warnings are problematic (do you see the warning if you're running gobin in a script, for example?) How about failing, and requiring a flag to force it to install anyway. Possible spellings: -f, -unstable, -force, -allow-unstable, ...