Stanza.WriteTo: Sort extra fields alphabetically

[wmanley]

Description of the Change

This makes the output of Stanza.WriteTo deterministic. This is important to me as I am using Packages index files as a kind of lockfile and committing it to my git repository. Without this we get a lot of noise in the diff whenever the file is regenerated because go randomises map iteration order.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable) - N/A
• bash completion updated (if applicable) - N/A
• documentation updated - N/A
• author name in AUTHORS

[wmanley]

I've not yet written a test for this change. I was hoping to expand on existing tests for canonicalOrder but I've not found them. I'm very much a golang novice but if someone can point me to the right place I'm sure I can figure it out. In theory the code should be more testable after this change as it's deterministic.

[wmanley]

Build passed on Travis, but seems to have failed to report the status back to github.

[smira]

👍 lgtm

[smira]

I've submitted #807 which fixes linting errors (should allow this PR to build).

[codecov]

Codecov Report

Merging #803 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage   64.11%   64.09%   -0.02%     
==========================================
  Files          51       51              
  Lines        6501     6512      +11     
==========================================
+ Hits         4168     4174       +6     
- Misses       1825     1831       +6     
+ Partials      508      507       -1

Impacted Files	Coverage Δ
deb/format.go	89.23% <100%> (+0.52%)	⬆️
query/lex.go	84.37% <0%> (-2.73%)	⬇️
deb/remote.go	62.74% <0%> (-0.14%)	⬇️

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[codecov]

Codecov Report

Merging #803 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
+ Coverage   64.19%   64.23%   +0.03%     
==========================================
  Files          51       51              
  Lines        6508     6514       +6     
==========================================
+ Hits         4178     4184       +6     
  Misses       1825     1825              
  Partials      505      505

Impacted Files	Coverage Δ
deb/format.go	89.23% <100%> (+0.52%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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