Use the same naming strategy as the raw packager

[pierredavidbelanger]

This fixes the HTML bundle name (on the filesystem) when its not the entry point.

This is one of the two points (required to be able to do require('./other.html')) mentioned in this comment

[codecov-io]

Codecov Report

Merging #1016 into master will decrease coverage by 0.48%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1016      +/-   ##
==========================================
- Coverage   89.06%   88.58%   -0.49%     
==========================================
  Files          68       68              
  Lines        3438     3258     -180     
==========================================
- Hits         3062     2886     -176     
+ Misses        376      372       -4

Impacted Files	Coverage Δ
src/packagers/HTMLPackager.js	95.74% <87.5%> (-1.76%)	⬇️
src/transforms/posthtml.js	80.48% <0%> (-19.52%)	⬇️
src/visitors/dependencies.js	76.62% <0%> (-10.39%)	⬇️
src/assets/ReasonAsset.js	91.66% <0%> (-8.34%)	⬇️
src/transforms/babel.js	87.37% <0%> (-6.8%)	⬇️
src/assets/CSSAsset.js	86.08% <0%> (-4.35%)	⬇️
src/assets/HTMLAsset.js	85.1% <0%> (-2.13%)	⬇️
src/utils/installPackage.js	63.49% <0%> (-1.59%)	⬇️
src/utils/getTargetEngines.js	95.5% <0%> (-1.13%)	⬇️
src/assets/JSAsset.js	90.83% <0%> (-0.77%)	⬇️
... and 13 more

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