New fix for `multiple_library_directives`

[FMorschel]

We should create a fix for multiple_library_directives that removes the line with the warning.

We should probably take into consideration any doc comments above it to merge with the original library directive, even more so with doc-imports. So maybe add three options for this fix:

• Remove directive (when no docs in either directive)
• Merge comments below
• Merge comments above

CC @bwilkerson

[bwilkerson]

How often do we think that users see this diagnostic? (I honestly don't have any data to answer that question, but wish that I did.) It's hard for me to imagine that it would be seen a lot. Together with the complexity of the fix and how easy it is to remove the extra directive manually I have to question the value of adding such a fix.

[FMorschel]

My main concern here is simply because of doc-imports. If people are copy-pasting a whole library into another one and have doc-imports, that could be a case for this fix. To help users not lose doc-imports when doing that. But yes, I see this is probably rare.