typescript: let user provide ParseConfigHost for config parse
typescript: let user provide ParseConfigHost for config parse

Summary:
We provided a pluggable file-reading function, but then later used
ts.sys (which goes back to the file system) in the next call.  Instead
consistently use a single parameter that injects all the dependencies.

Reviewers: #core_team, salguarnieri

Reviewed By: #core_team, salguarnieri

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1600

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