fix(ingestion): Looker original view name should be used for explore_joins

[sebkim]

Looker Explore consists of Looker Views.
For example,

explore: revenue {
  description: "Some description" 
  view_name: opportunity
  ...

  join: feature_enablement {
    view_label: "Current Quota and Feature Enablement"
    from: app_subscription
    sql_on: ${feature_enablement.billing_organization_id}=${app_organization_billing.billing_organization_id}
            and ${feature_enablement.applied} and ${feature_enablement.current} and coalesce(${feature_enablement.product},'')<>'SUPPORT'
            and not(${feature_enablement._fivetran_deleted})
      ;;
    type: left_outer
    relationship: one_to_one
    fields: []
  }
}

revenue explore's base view is opportunity and it joins with feature_enablement. Here, feature_enablement is just the name of the join and its original view name is app_subscription.

Current issue of the Looker ingestion in the case of join has from attribute is the upstream metadata of the explore (e.g; revenue explore) does not use the original view name (e.g; app_subscription) but use the join name (e.g; feature_enablement). Join name dataset entity is just empty.

In this PR, I made a fix for the above issue.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[github-actions]

Unit Test Results (build & test)

341 tests  ±0   341 ✔️ ±0   3m 22s ⏱️ +26s
  83 suites ±0       0 💤 ±0 
  83 files   ±0       0 ❌ ±0 

Results for commit aa21e52. ± Comparison against base commit 538cfba.

♻️ This comment has been updated with latest results.

[github-actions]

Unit Test Results (metadata ingestion)

       5 files  ±0         5 suites  ±0   1h 34m 15s ⏱️ + 5m 59s
   555 tests ±0     552 ✔️ ±0    3 💤 ±0  0 ❌ ±0 
2 552 runs  ±0  2 477 ✔️ ±0  75 💤 ±0  0 ❌ ±0 

Results for commit aa21e52. ± Comparison against base commit 538cfba.

♻️ This comment has been updated with latest results.

[shirshanka]

LGTM! Thanks for the contrib @sebkim 🙏