place all definitions at the same level

[rogpeppe]

The JSON Schema draft-06 says of the definitions keyword:

This keyword's value MUST be an object. Each member value of this object MUST be a valid JSON Schema.

In the buildkite schema, the definitions/commonOptions property is used
to namespace schemas, not as a schema itself, which doesn't seem
to fit the intended structure for definitions, even though technically it's just about
OK because none of the properties happen to coincide with keywords
defined in draft-06 ("if" was only defined in draft-07) and unknown keywords
are ignored.

Referring to definitions that aren't in regular schema locations is also
in a grey area. Later versions of the spec have this to say:

a reference target under a known keyword, for which the value is known not to be a schema, results in undefined behavior

Even though draft-06 doesn't include this wording, it seems that the reasoning
still applies.

So to avoid the above issues, this PR moves all the definitions that
were inside commonOptions into the top level definitions property.

The distinction between the two kinds of definitions is still clear by virtue
of the Step suffix used for all the actual step definitions and not for
the others.

Some other possibilities:

1. give all the schemas inside commonOptions a common prefix, such as "common.", so we'd have #/definitions/common.env etc.
2. move all the definitions inside commonOptions one level down into another definitions property, so we'd have #/definitions/commonOptions/definitions/env etc.

Both of those seem unneccessarily verbose to me, but YMMV.

I have verified that the tests pass with this change in place,
assuming #83 is landed as a prereq.

I've split this PR into two commits to make the diffs easier to see.
The first one leaves the indentation inplace; the second aligns it correctly.