Convert ListVersions() to return []PairedVersion

[sdboyer]

This is almost complete, except one test is still failing in a way that
indicates the test harness still isn't quite dealing with the underlying
FAKEREVs correctly.

Fixes #202.

[houndci-bot]

can probably use "var pvl []PairedVersion" instead

[sdboyer]

An enterprising soul could probably figure this one out and update this issue with a suggestion about where the problem is 😄

The goal within the test harness - in depspecSourceManager.ListVersions() - is to produce those paired versions with an underlying FAKEREV only if necessary to meet the interface requirement, and then strip them out immediately in depspecBridge.listVersions(). The latter is the only thing the solver should be calling...I think. Somehow, though, FAKEREVs are making it through into the solutions. The task here is to figure out where that leak is.

[codecov]

Codecov Report

Merging #210 into master will decrease coverage by 3.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   78.02%   74.83%   -3.19%     
==========================================
  Files          28        4      -24     
  Lines        4018      465    -3553     
==========================================
- Hits         3135      348    -2787     
+ Misses        659       95     -564     
+ Partials      224       22     -202

Impacted Files	Coverage Δ
selection.go
source_cache.go
deduce.go
rootdata.go
maybe_source.go
cmd.go
vcs_source.go
constraints.go
trace.go
manifest.go
... and 10 more

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[codecov]

Codecov Report

Merging #210 into master will increase coverage by 0.39%.
The diff coverage is 96.79%.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   79.01%   79.41%   +0.39%     
==========================================
  Files          30       31       +1     
  Lines        4175     4143      -32     
==========================================
- Hits         3299     3290       -9     
+ Misses        649      624      -25     
- Partials      227      229       +2

Impacted Files	Coverage Δ
satisfy.go	80.92% <100%> (ø)	⬆️
version.go	82.36% <100%> (+3.29%)	⬆️
solver.go	83.97% <100%> (+0.09%)	⬆️
bridge.go	65.38% <100%> (-10.54%)	⬇️
version_queue.go	100% <100%> (ø)	⬆️
selection.go	82.05% <100%> (ø)	⬆️
source_manager.go	91.23% <100%> (+0.88%)	⬆️
version_unifier.go	95.61% <95.61%> (ø)
pkgtree/pkgtree.go	81.4% <0%> (-3.67%)	⬇️
... and 1 more

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