Only detect plugins when they need to exist in global

[watsonjj]

As first described in #1035, I am having trouble with the following issue with the io plugins, specifically regarding the TargetIOR1Calibrator:

(cta) 09:27 [Jason ~/Software/ctapipe_io_targetio]  (master)  $ python -c "from ctapipe.calib.camera.r1 import CameraR1Calibrator"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/Jason/Software/ctapipe/ctapipe/calib/__init__.py", line 5, in <module>
    from .camera import *
  File "/Users/Jason/Software/ctapipe/ctapipe/calib/camera/__init__.py", line 8, in <module>
    from .dl1 import *
  File "/Users/Jason/Software/ctapipe/ctapipe/calib/camera/dl1.py", line 14, in <module>
    from ...image import NeighbourPeakIntegrator, NullWaveformCleaner
  File "/Users/Jason/Software/ctapipe/ctapipe/image/__init__.py", line 1, in <module>
    from .hillas import *
  File "/Users/Jason/Software/ctapipe/ctapipe/image/hillas.py", line 11, in <module>
    from ..io.containers import HillasParametersContainer
  File "/Users/Jason/Software/ctapipe/ctapipe/io/__init__.py", line 12, in <module>
    detect_and_import_io_plugins()
  File "/Users/Jason/Software/ctapipe/ctapipe/core/plugins.py", line 16, in detect_and_import_io_plugins
    return detect_and_import_plugins(prefix='ctapipe_io_')
  File "/Users/Jason/Software/ctapipe/ctapipe/core/plugins.py", line 10, in detect_and_import_plugins
    in pkgutil.iter_modules()
  File "/Users/Jason/Software/ctapipe/ctapipe/core/plugins.py", line 11, in <dictcomp>
    if name.startswith(prefix)
  File "/Users/Jason/anaconda3/envs/cta/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/Users/Jason/Software/ctapipe_io_targetio/ctapipe_io_targetio/__init__.py", line 15, in <module>
    from .calibration import TargetIOR1Calibrator
  File "/Users/Jason/Software/ctapipe_io_targetio/ctapipe_io_targetio/calibration.py", line 1, in <module>
    from ctapipe.calib import CameraR1Calibrator
ImportError: cannot import name 'CameraR1Calibrator' from 'ctapipe.calib' (/Users/Jason/Software/ctapipe/ctapipe/calib/__init__.py)

It happens because when importing the CameraR1Calibrator, the __init__.py files end up leading to import the io plugins, which in turns tries to import the TargetIOR1Calibrator, which requires CameraR1Calibrator to be defined.

One solution I have is to be a bit more careful about when we import the plugins, instead only importing when we need them to exist in the global namespace. I.e. when we are using the factory methods.

[maxnoe]

While I think this is a good short-term fix, we should also do what I proposed in #1035 and disentangle our subpackages more.

[codecov]

Codecov Report

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The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1036   +/-   ##
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  Coverage          ?   83.52%           
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  Files             ?      188           
  Lines             ?    10658           
  Branches          ?        0           
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  Hits              ?     8902           
  Misses            ?     1756           
  Partials          ?        0

Impacted Files	Coverage Δ
ctapipe/io/__init__.py	100% <ø> (ø)
ctapipe/io/eventsource.py	98.11% <100%> (ø)
ctapipe/core/component.py	96.55% <100%> (ø)

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[dneise]

I like this.
I believe, I sometimes tend to put too much 'function' into init files, which can be extremely surprising ...

So moving stuff out there might actually be the right choice here ... also longterm ...

[watsonjj]

@maxnoe Would you be happy with approving this?

[maxnoe]

A sorry, yes, the comment was meant to be the approval, missed the correct radio selection