Using unique_id for manifest and catalog nodes to allow for aliased m…

[vvvito]

Resolves #30
Potentially resolves #40

What has changed?

Updates to the load_manifest and load_catalog functions to use the unique_id of the dbt model or test, instead of a schema + aliased name of the object.

The Coverage Report still returns the aliased object name, but the documentation coverage will be correctly calculated for each table as the manifest and catalog now use the normalized unique_id

Tested this for both doc and test cov_type and working as expected.

[mrshu]

Thanks @vvvito, this does indeed look like it might work!

@sweco, would you mind taking a look as you were the architect of the approach that's currently being used?

Thanks!