introspector: use introspector to get harness/source pairs by default

[DavidKorczynski]

Ref: #525

[DavidKorczynski]

This performs a lot better than our existing automated setup, but there are still three discrepancies between the manually generated ones from #524:

"project": "librawspeed"
"target_name": "/src/librawspeed/fuzz/librawspeed/codes/PrefixCodeDecoder/Dual.cpp"
"target_path": "/src/librawspeed/fuzz/librawspeed/codes/PrefixCodeDecoder/Dual.cpp"


"target_name": "fuzztest"
"target_name": "fuzztest_proto2_static"
"target_path": "/src/nanopb/tests/fuzztest/fuzztest.c"


"project": "proftpd"
"target_name": "fuzzer"
"target_path": "/src/fuzzer.c"

These will be dealt with on the introspector side -- but this still performs a lot better than the current status quo -- this is good to go.

Finally, there are some benign discrepancies to #524 in that sometime the current heuristic will select other harnesses than the manually selected ones. In general, this is no problem. However, there are some potential cases where we may want to select a specific harness within a given project if the given harness has a wider (or different) linkage. An example here is abseil-cpp where the better harness to taget is string_utilities_fuzzer as it includes more sources from the source tree: https://github.com/google/oss-fuzz/blob/master/projects/abseil-cpp/BUILD

[DavidKorczynski]

/gcbrun skip