Optimize import handling (#14115)

Instead of processing all imports as a separate pass before
the rest of the stylesheet, process them as any other node.

Change-Id: I26631edda78e76374c91c2e8767db50f88cb99a9
Optimize scopes by reducing map creation (#14116)

Change-Id: I2c59e8d1ea6aeea8706d26f0af13bf35ed19b9e8