Fix aliasing with optional tokens

[dennisgl]

Previously to alias an optional dependency of a plugin, one would have to alias the "optional type token" given the alias map key was TokenImpl instead of Ref().

app
  .register(PluginWithOptionalDep)
  .alias(OptionalDep.optional, AlternativeDep);
app.register(AlternativeDep, AlternativePlugin);

->

app
  .register(PluginWithOptionalDep)
  .alias(OptionalDep, AlternativeDep);
app.register(AlternativeDep, AlternativePlugin);

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[codecov[bot]]

Codecov Report

Merging #335 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files          18       18           
  Lines         443      443           
  Branches       86       86           
=======================================
  Hits          415      415           
  Misses         15       15           
  Partials       13       13

Impacted Files	Coverage Δ
src/base-app.js	91.08% <100%> (ø)	:arrow_up:

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[dennisgl]

!merge