fogpy can't implicitly load dependencies

[gerritholl]

import glob
import sattools.ptc
from satpy import Scene
fn_sev = glob.glob("/media/nas/x21308/scratch/SEVIRI/2013/12/12/*")
fn_cmsaf = ["/media/nas/x20629/SEVIRI/repr2/level2/cpp/2013/12/12/CPPin20131212100000305SVMSG01MD.nc"]
reader_sev = "seviri_l1b_hrit"
reader_cmsaf = "cmsaf-claas2_l2_nc"
sc = Scene(filenames={
            reader_sev: fn_sev,
            reader_cmsaf: fn_cmsaf})
sattools.ptc.add_pkg_comps_mods(sc, "fogpy", sensors=["seviri"])
ls = sc.resample("germ2")
ls.load(["fls_day"])
ls.show("germ2")

fails with

  File "mwe53.py", line 13, in <module>
    ls.load(["fls_day"])
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 971, in load
    raise KeyError("Unknown datasets: {}".format(unknown_str))
KeyError: 'Unknown datasets: VIS006, IR_120, IR_087, VIS008, IR_108, IR_039, IR_016'

It works when adding

sc.load(["VIS006", "IR_120", "IR_087", "VIS008", "IR_108", "IR_039", "IR_016", "cot", "cwp", "reff"])

The need for explicit loading is partly an inevitable consequence of the workaround in https://github.com/pytroll/fogpy/commit/1e85cd3af3c0c9557b719c0702d839d335b242c7, but that doesn't explain why it also finds the datasets it can find with SEVIRI unknown. This may be an issue with satpy dependencies not found when having multiple readers?