Bug 951736 - Adjust sys.path while native commands are executed; r=ted

The previous behavior only adjusted sys.path during module import. This
caused problems when there were delayed imports.

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extra : rebase_source : ca331f6da5f4e69ae74e8e831a41b12405fdd7fc
extra : amend_source : e0bd180a6fe5b5f64b2b8860f85887a9fd5b848e
Bug 949875 - Delay import some WebIDL modules; r=glandium

This prevents excessive dependencies on config.status. Those extra
dependencies make WebIDL developer workflow inefficient.

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extra : rebase_source : d656410da544d4d29eb926293d8be355cd3736db