[SPARK-12152][PROJECT-INFRA] Speed up Scalastyle checks by only invoking SBT once

[JoshRosen]

Currently, dev/scalastyle invokes SBT four times, but these invocations can be replaced with a single invocation, saving about one minute of build time.

[vanzin]

I have code in #6502 to keep the scalastyle.txt file around (so we can look at warnings). Both changes will conflict in any case, but it would be nice to have the complete output available.

[JoshRosen]

@vanzin, what about just running sbt/sbt scalastyle by hand if you want to inspect the log output?

[vanzin]

Either way. It just seems nice to keep the whole output around when it's so cheap to do it.

[SparkQA]

Test build #47209 has finished for PR 10151 at commit 9de4d4c.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public abstract static class PrefixComputer\n

[JoshRosen]

We can handle retention of the output in a followup, since it'd probably be best to also do it for Java, Python, and R style checks. Therefore, I'm going to merge this now.