[CFE] Fix incremental compiler bug when library claimed to have other library as part

This CL fixes a bug where the incremental compiler could potentially
* invalidate to few builders,
* output too few libraries, or
* crash

if one library claimed to have another (stand-alone) library as part.

This could happen when the library claimed to be a part was invalidated,
though it wouldn't always because it is sensitive to ordering.

The issue is that the library claiming to have another library as a part
lists that library as its part. Normally such a "pair" is only one
library and thus only has one LibraryBuilder (when from dill). Because
it's actually two libraries though, there are actually two LibraryBuilders.
Under normal circumstances we map from the part uri to the "main builder"
but in this case that could (depending on ordering) override the actual
builder which caused the bug.

Change-Id: Iff9037575ff53b48126b20f9fc5a6c8a52ed5198
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176241
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>