[BEAM-14374] Fix module import error in FullyQualifiedNamedTransform

[ihji]

The second argument of import_module is ignored when the first argument is not a relative name. For example, the current implementation works for io module since import_module('io') also works however it doesn't work for apache_beam.dataframe since import_module('dataframe') also doesn't work.

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[ihji]

R: @robertwb

[codecov]

Codecov Report

Merging #17482 (38ce39f) into master (07f30d2) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17482      +/-   ##
==========================================
+ Coverage   73.93%   74.02%   +0.09%     
==========================================
  Files         689      691       +2     
  Lines       90397    90873     +476     
==========================================
+ Hits        66832    67266     +434     
- Misses      22381    22423      +42     
  Partials     1184     1184              

Flag	Coverage Δ
python	83.71% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...beam/transforms/fully_qualified_named_transform.py	100.00% <100.00%> (ø)
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
...s/interactive/dataproc/dataproc_cluster_manager.py	84.21% <0.00%> (-3.70%)	⬇️
sdks/python/apache_beam/coders/row_coder.py	94.49% <0.00%> (-2.51%)	⬇️
sdks/python/apache_beam/runners/common.py	87.94% <0.00%> (-2.48%)	⬇️
sdks/python/apache_beam/io/gcp/gcsfilesystem.py	88.23% <0.00%> (-1.77%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	89.20% <0.00%> (-1.64%)	⬇️
sdks/python/apache_beam/io/localfilesystem.py	90.97% <0.00%> (-1.28%)	⬇️
...ive/messaging/interactive_environment_inspector.py	96.66% <0.00%> (-1.21%)	⬇️
sdks/python/apache_beam/dataframe/frame_base.py	89.54% <0.00%> (-0.83%)	⬇️
... and 46 more

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[robertwb]

Can you add a test?

[ihji]

@robertwb added a test

[robertwb]

Sorry to be a stickler, but this mocking doesn't actually convince me that the code in question actually does the right thing. Could you actually perform an import here so that we know we're interacting with the import module in the right way?

[ihji]

Other tests already cover some real importing but I added two more that would have failed without the change in this PR.

[robertwb]

Perfect. Thanks!