Suggestion: Warn when a package was packaged incorrectly

[matthid]

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.

[matthid]

Regarding the last point: You could even argue that paket resolved an invalid/broken resolution... @forki what do you think?