This can find, read, and merge them. It handles `include` inside the
file.

It works using an abstraction over the file system so that (in theory at
least), this code could be harvested and made reusable by other tools
that need to work with analysis_options.yaml files but don't want to
work directly with files on disk.

It also includes an implementation of that abstraction using dart:io.

Right now, this is all just private inside dart_style, but is organized
so that it would be easy to pull out into a separate package later if we
want.

It was getting pretty huge. I didn't want to stuff more tests in there
for CLI integration of analysis_options, so splitting it up into smaller
files first.

There are no meaningful changes here, just moving code around.

When formatting from the CLI using files or stdin with a --stdin-name
option, looks in the surrounding directories for an
analysis_options.yaml file. If found and the file specifies a formatter
page width, uses that.

Handles includes and merging as well. Doesn't handle "package:" includes
yet.

# Conflicts:
#	lib/src/language_version_cache.dart