[Bugfix] Avoid symbol conflicts in MakePackedAPI/MakeUnpackedAPI

[Lunderberg]

PRs #14913 and #14914 made analogous changes to MakePackedAPI and MakeUnpackedAPI to handle subroutine calls. Both PRs introduced the same symbol, tvm::tir::SubroutineCallRewriter, a local utility to update internal calls to a modified function. While each PR passed CI individually, and was therefore able to merge, having both changes caused a duplicate symbol.

This commit updates MakePackedAPI and MakeUnpackedAPI to place their local utilities into anonymous namespaces, avoiding the conflict.

[tvm-bot]

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[tqchen]

does this hint perhaps we should have a common utility somehow as internal header, looking at, given this is small it is likely ok

[csullivan]

Thanks @tqchen. Yeah, had the same thought but perhaps we can remove MakeUnpackedAPI altogether later.

In discussing with @Lunderberg it seems the special case handling MakeUnpackedAPI introduces for AOT should be able to be removed once annotations are used for whether a function requires packing or not. MakePackedAPI could then code generate the packed api when the functions are annotated and AOT functions would remain unpacked as they lack the annotation.