Direct import of tar files into virtual directories

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/521)
In GitLab by [[Gitlab user @jmacarthur]](https://gitlab.com/jmacarthur) on Jul 27, 2018, 11:27

## Background

This is an issue filed for a proposed MR !445 which currently extracts tar files to a temporary directory before importing that temporary directory. This is inefficient.

## Task description

* [ ] Implement direct tar file import into virtual directories

## Acceptance Criteria

Direct import of tar files with functional equivalence to existing code.

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Start with the original GitLab issue and proposed MR !445, then trace the existing code path that extracts tar files into a temporary directory before import. Done means tar files are imported directly into virtual directories while retaining functional equivalence with the existing behavior.

Written by the indexing model from the issue text.