Handle promises with unavailable references if resolve=False

[ines]

If we're just filling a config, promises that refer to registered functions that are not available should be ignored.

[codecov]

Codecov Report

Merging #400 into master will increase coverage by 0.03%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
+ Coverage   59.71%   59.74%   +0.03%     
==========================================
  Files          96       96              
  Lines        6622     6630       +8     
==========================================
+ Hits         3954     3961       +7     
- Misses       2668     2669       +1     

Impacted Files	Coverage Δ
thinc/config.py	96.04% <90.00%> (-0.13%)	⬇️
thinc/about.py	100.00% <100.00%> (ø)

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