fuzzers: rename "fuzz" directory to match our style
Our layout uses names like "examples" or "tests" which is why the "fuzz"
directory doesn't really fit in here. Rename the directory to be called
"fuzzers" instead. Furthermore, we rename the fuzzer "fuzz_packfile_raw"
to "packfile_raw_fuzzer", which is also in line with the already
existing fuzzer at google/oss-fuzz.
While at it, rename the "packfile_raw" fuzzer to instead just be called
"packfile" fuzzer.
