Make aliasing subquery configurable for render_limited

[colin-rogers-dbt]

partially addresses #124

Currently only a subset of adapters require adding an alias to the subquery generated by render_limited (i.e. when a user passes the --empty flag). Currently if a user has already aliased a referenced model or source we duplicate the aliasing and generate invalid SQL.

Unfortunately at the moment we have no way of knowing if a user has already aliased the reference so quickest mitigation is to simply not add the alias if the platform does not require it. This should allow us to fix this bug for many platforms (snowflake, bigquery) but doesn't fix this for others (postgres, mysql, oracle).

dbt-snowflake update: dbt-labs/dbt-snowflake#980

docs dbt-labs/docs.getdbt.com/#

Problem

Solution

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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