Module not found "fuzzers.xxxxx" upon testing with new fuzzer

I add a new fuzzer following instructions from https://google.github.io/fuzzbench/getting-started/adding-a-new-fuzzer/.
Basically, this new fuzzer directory (I call hfuzz_rb_afl) is a copy of aflplusplus_qemu.

If I try to test build the fuzzer

```bash
export FUZZER_NAME=hfuzz_rb_afl
cd fuzzbench_git_repo
source ./.venv/bin/activate
export PYTHONPATH=.
make -j4 build-$FUZZER_NAME-all
```

I get
```
=> ERROR [stage-1 15/15] RUN echo "Run fuzzer_build to build the target" && if [ -z "$debug_builder" ] ; then fuzzer_build; fi 4.0s
------
> importing cache manifest from gcr.io/fuzzbench/builders/hfuzz_rb_afl/bloaty_fuzz_target:
------
------
> [stage-1 15/15] RUN echo "Run fuzzer_build to build the target" && if [ -z "$debug_builder" ] ; then fuzzer_build; fi:
1.145 Run fuzzer_build to build the target
1.225 FUZZ_TARGET = fuzz_target
1.225 CFLAGS = -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -pthread -Wl,--no-as-needed -Wl,-ldl -Wl,-lm -Wno-unused-command-line-argument -O3
1.225 CXXFLAGS = -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -pthread -Wl,--no-as-needed -Wl,-ldl -Wl,-lm -Wno-unused-command-line-argument -stdlib=libc++ -O3
1.226 Traceback (most recent call last):
1.226 File "", line 1, in
1.226 ModuleNotFoundError: No module named 'fuzzers.hfuzz_rb_afl'
------

3 warnings found (use --debug to expand):
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 26)
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 27)
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 28)
Dockerfile:66
--------------------
64 |
65 | COPY docker/benchmark-builder/fuzzer_build /usr/bin/fuzzer_build
66 | >>> RUN echo "Run fuzzer_build to build the target" && if [ -z "$debug_builder" ] ; then fuzzer_build; fi
67 |
--------------------
ERROR: failed to solve: process "/bin/sh -c echo \"Run fuzzer_build to build the target\" && if [ -z \"$debug_builder\" ] ; then fuzzer_build; fi" did not complete successfully: exit code: 1
make: *** [docker/generated.mk:36824: .hfuzz_rb_afl-bloaty_fuzz_target-builder] Error 1
```

Note that I do set the PYTHONPATH beforehand and execute the above in the venv.

Would really appreciate some help here :)

PS: I am completely local and this new dummy fuzzer is only for test, not to outsource as of yet