Suggestion: Warn when a package was packaged incorrectly

Description

In Short: Add a big red warning when stuff like https://github.com/fsharp/fsharp/issues/672 happens.

In general paket update/install (or maybe even restore) could detect this whole class of packaging errors (even earlier when using pack)

• Warn when a binary is part of an incompatible framework folder. (the above issue)
• Warn when the package contains a dependency group but no folder for the corresponding framework (it should at least contain an empty folder)
• Warn when a dependency was not added because no group was matched (and therefore empty dependencies are assumed). For example see https://www.nuget.org/packages/FsCheck.xunit . If you use net45 and reference FsCheck.xunit you will get compiler errors because xunit.abstractions will not be referenced (because the dependency is for group net452 only)

My suggestion would be to add those warnings and see how often they trigger, if they trigger to often we might want to disable them or add some kind of "pure" mode.

Start by tracing the paket update, install, restore, and possibly pack entry points mentioned in the issue. Review how framework folders and dependency groups are handled, then define tests covering the three listed packaging-error cases. Done means the relevant commands warn about these mismatches without silently accepting missing dependencies.