Add CollectionIndexOnNonIndexedSeq rule to Scapegoat

[JoshRosen]

Inspired by https://twitter.com/jshrsn/status/865677863646658560, this patch adds a new IndexingOnANonIndexedSeq rule which raises a warning when we see code which indexes on a Seq which isn't known to be an IndexedSeq subtype. The goal here is to make it easier to prevent cases where someone writes an O(n^2) loop by accident.

This rule should have prevented the following Spark bugs:

• [SPARK-20773][SQL] ParquetWriteSupport.writeFields is quadratic in number of fields  apache/spark#18005
• [SPARK-9952] Fix N^2 loop when DAGScheduler.getPreferredLocsInternal accesses cacheLocs apache/spark#8178
• [SPARK-17648][CORE] TaskScheduler really needs offers to be an IndexedSeq apache/spark#15221

For comparision, here's the same rule implemented in linter: JoshRosen/linter#1

[JoshRosen]

Upstream PR: scapegoat-scala#158