Support composer-manager properly

[LionsAd]

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https://github.com/drupal-media/url_embed/pull/6 should not be needed for each project.

Support properly via an option within DRUPAL_TI:

# Auto-detect if composer.json with type: 'drupal-module' is present
# Use 0 to disable, use 1 to enable.
DRUPAL_TI_USE_COMPOSER_MANAGER="auto"

Needs to be supported by simpletest and behat runners.

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@davereid: ^^

[davereid]

Note that using D7 composer_manager works differently than D8 composer_manager. Would this need to be taken into account?