Reduce summarization API to one method

[EgorkaKulikov]

Description

We had two very similar methods in summarization API, and one of them had implicit and long sources analysis.
A unique method with explicit logic is introduced instead of them.

How to test

Automated tests

Our pipeline will do all auomated checks itself.

Manual tests

Standard summaries checks in utbot-plugin.

Self-check list

Check off the item if the statement is true:

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.