Improve error reporting: importing modules from an assembly compiled as Exe but without explicit EntryPoint should raise a warning

## what

F# users not acquainted with module static initialization semantics in exe project (which apparently differ from library project) might expect a different result when doing this:
## steps

``` shell
C:\tmp\repro>more main.fs
```

``` fsharp
module Foo
type Buildable = Buildable of obj
let o = Buildable null
```

``` shell
C:\tmp\repro>fsc main.fs
Microsoft (R) F# Compiler version 14.0.23413.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

C:\tmp\repro>more test.fsx
```

``` fsharp
#r @"main.exe"
printfn "%A" Foo.o
```

``` shell
C:\tmp\repro>fsi test.fsx

C:\tmp\repro>
```
## Actual results

The static type initializer for module Foo wasn't executed and `Foo.o` will always remain null.
## Expected results

If we want to preserve that behaviour, I think a warning when referencing a .exe containing F# modules would help user to understand that modules might not be initialized as intended.

If we could fix the behaviour while maintaining compatibility / non breaking change, that would IMHO be ideal.