Upgrade/Install: Select the main plugin file after installation

[ArkaPrabhaChowdhury]

Trac ticket: https://core.trac.wordpress.org/ticket/22287

This prefers a root-level plugin file matching the plugin directory name, then falls back to the first root-level plugin file so embedded plugin headers do not produce an incorrect activation link.

Tests: phpunit tests/phpunit/tests/admin/pluginUpgrader.php; PHPCS on both changed files.

[github-actions]

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[github-actions]

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