Rename "External Matchfinder" to "Block-Level Sequence Producer"

[embg]

"External matchfinder" was an inaccurate name, since the user-provided callback selects which match to use at each position as well as finding multiple matches for each position. The updated term we are using is "sequence provider".

Original PR introducing the old term (title of the old PR has been updated): #3333

Here are the commands I used to migrate names in lib/ and tests/:

find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/externalMatchState/sequenceProducerState/g' {} \;
find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/ExternalMatchState/SequenceProducerState/g' {} \;
find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/externalMatchFinder/sequenceProducer/g' {} \;
find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/ExternalMatchFinder/SequenceProducer/g' {} \;
find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/ZSTD_EXTERNAL_MATCHFINDER_ERROR/ZSTD_SEQUENCE_PRODUCER_ERROR/g' {} \;
find ./ -type f \( -name "*.c" -o -name "*.h" \) -exec sed -i 's/ZSTD_c_enableMatchFinderFallback/ZSTD_c_enableSeqProducerFallback/g' {} \;

TODOs:

• Update all public headers and contrib/externalMatchfinder
• Update fuzzer makefile (references contrib)
• Open an issue to track migration of internal names / comments in lib/ and tests/. Not a release blocker for 1.5.4 since it's all internal.