replay: Prevent external path removal when defined on multiple dirs

The same absolute external path can be defined on multiple directories in the repository.

To prevent removal of an external path while it is still defined in another directory, we need to track the number of references of the absolute external path by incrementing / decrementing a counter when adding / removing an external.

Related to #611

Depends on !121 (closed)

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Migrated from D7127 (view on Phabricator)