Fixes undefined index error and adds new helper method (#825)

[p810]

Description

• Adds Migration\Step\Eav\Helper::getKeyFromFields() to consolidate the functionality used to assemble a field key for a row, giving the action semantic meaning and making the code a little DRYer
• Fixes the undefined index error that's thrown from Migration\Step\Eav\Data::createMapAttributeSetIds(), by adding a new property $mapNewAttributeSetsToSourceKeys which maintains a mapping of source to destination field keys (e.g. 1-Default => 1-Migration_Default)

Fixed Issues (if relevant)

1. magento/data-migration-tool#825: Undefined index: 1-Default in Migration\Step\Eav\Data on line 624

Manual testing scenarios

1. Run the data migration tool for a migration of Magento 1.14.4.5 to 2.3.5-p1
2. Ensure that an exception representing an undefined index notice isn't thrown from Migration\Step\Eav\Data::createMapAttributeSetIds()

Further information can be found in #825.

Contribution checklist

• [x] Pull request has a meaningful description of its purpose
• [x] All commits are accompanied by meaningful commit messages
• [x] All new or changed code is covered with unit/integration tests (if applicable)

[victor-v-rad]

Thank you @p810 for your contribution to the project. Internal ticket MC-35383 to process it.

[p810]

As @victor-v-rad mentioned in #825 this isn't a bug but was caused by a handler that got leftover in our configuration.

[ghost]

Hi @p810, thank you for your contribution!
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