Improve the error when imports are written before a `part of` directive

[DanTup]

With enhanced parts, it seems imports should be before part directives, but they come after part of directives. This makes some sense, but it's easy to get wrong. If I put an import after a part, I get a clear error message:

However the same is not true if I get a part of and import the wrong way around:

The error makes it seem like I can't have imports in this file, but in reality I just need to put them after the part of.

[FMorschel]

This should also take the library directive into account. When we add multiple in a library, we trigger multiple_library_directives, but when adding either one or many library directives in a part file, we get the same non_part_of_directive_in_part.

[bwilkerson]

Agreed. There's a lot of work that needs to be done before we can ship the enhanced-parts feature, but we aren't actively working on that feature at the moment.