Let `reuse` look inside git submodules
Changes `collect-license-metadata` and `generate-copyright` so they can now look at the git submodules.
Unfortunately `reuse` chokes on the LLVM submodule - it finds the word "Copyright" or the unicode copyright symbol in all kinds of places, including UTF-8 test cases. The `reuse` tool expressly won't let you ignore folders, so we let it scan everything and then strip out the LLVM sub-folder in post. Instead, we add in a hand-curated list of copyright information gleaned by reading the LLVM codebase carefully, which is stored in `.reuse/dep5` in Debian format where `reuse` can find and use it.
The `.reuse/dep5` continues to track copyright info for files in the tree that do not have SPDX metadata in them (i.e. all of them)
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