mstch-cpp2: Fix generation of includes with duplicated name
mstch-cpp2: Fix generation of includes with duplicated name

Summary: Previously, we had a bug in which thrift includes with duplicated names would get overwritten in the cache by the local namespace. The reason is because when generating these includes, we were using the current program namespace rather than the include namespace.

Reviewed By: yfeldblum

Differential Revision: D5754305

fbshipit-source-id: 72deea6bd000edea88d18d21cda6e858fa8ab549

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