Make dbt support documentation for snapshots and seeds (and analyses) (#1974)

[beckjake]

Fixes #1974

Adds threetwo new sections to schema.yml: snapshots, and seeds, and analyses. They act just like "models", except they must refer to resources of the appropriate type. dbt checks correctness at patch-time.

For backwards compatibility, this PR allows snapshots/seeds /analyses tests+docs to be defined under 'models', but with a deprecation warning. There's no warning for models described by "models", of course!

I did some minor internal refactoring of the schema.yml parser, by creating two custom parser classes to handle sources vs ... "refables" I guess you could call them? I imagine creating a third for macros in the future, with similar behavior but no support for "column tests", etc.

[drewbanin]

This looks really great! I did some functional testing here, and I really love the way this feels in practice. Some copy comments here, and a couple of other minor things to note:

If a test references a node that doesn't exist, dbt shows:

WARNING: Test 'test.my_project.unique_name_id' (models/schema.yml) depends on model 'name' which was not found

Can we swap out this text for something like:

WARNING: Test 'test.my_project.unique_name_id' (models/schema.yml) depends on a node named 'name' which was not found.

Minor, but more sensible with some of the changes taking place in this PR.

Also, looks like this code allows you to define columns for an analysis node. This is groovy, but can we disallow the specification of tests on those columns (and on the resource itself)? I can't think of a good reason to support the definition of tests on analyses, just insofar as dbt can't actually run them!

[drewbanin]

How about something like

'{node.name}' is a {node.resource_type} node, but it is specified in the
{patch.yaml_key} section of {node.original_file_path}.

To fix this error, place the `{node.name}` specification under the {patch.yaml_key} key instead.

[drewbanin]

"{node.name}" is a {node.resource_type} node, but it is specified in the
models section of {node.original_file_path}.

To fix this warning, place the `{node.name}` specification under the {patch.yaml_key} key instead.
This warning will become an error in a future release.

[beckjake]

Also, looks like this code allows you to define columns for an analysis node. This is groovy, but can we disallow the specification of tests on those columns (and on the resource itself)? I can't think of a good reason to support the definition of tests on analyses, just insofar as dbt can't actually run them!

I think I'm actually going to disable support for analysis nodes for now since that's extra work that nobody probably cares about? I'd need to make a new patch type that includes docs but not tests - I'll have to do similar for macros, so I'll look into it then.

[drewbanin]

Sounds good - makes sense to hit analysis docs at the same time as macro docs.

[drewbanin]

Some comments about the formatting with linewrap, but this will LGTM when that's addressed!

[drewbanin]

Looks great, ship it!