[SPARK-3873][BUILD] Add style checker to enforce import ordering.

The checker tries to follow as closely as possible the guidelines of
the code style document, and makes some decisions where the guide is
not clear. In particular:

- wildcard imports come first when there are other imports in the
  same package
- multi-import blocks come before single imports
- lower-case names inside multi-import blocks come before others

In some projects, such as graphx, there seems to be a convention to
separate o.a.s imports from the project's own; to simplify the
checker, I chose not to allow that, which is a strict interpretation
of the code style guide, even though I think it makes sense.

Since the checks are based on syntax only, some edge cases may
generate spurious warnings; for example, when class names start
with a lower case letter (and are thus treated as a package name
by the checker).

The checker is currently only generating warnings, and since there
are many of those, the build output does get a little noisy. The
idea is to fix the code (and the checker, as needed) little by little
instead of having a huge change that touches everywhere.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6502 from vanzin/SPARK-3873.

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