Add marked source for Go package nodes.
Add marked source for Go package nodes.

Summary:
With this commit, Go nodes are now given marked source facts corresponding to
their import paths. For compound import paths, the part before the final slash,
if any, is presented as context; the final component is the identifier.

Reviewers: zarko, #core_team

Reviewed By: zarko, #core_team

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D2184

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