Make Pkg.add("Foo") and import Foo two different steps

[DilumAluthge]

Fixes #114

[codecov]

Codecov Report

Merging #151 into master will increase coverage by 0.89%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   59.93%   60.83%   +0.89%     
==========================================
  Files          17       17              
  Lines         659      674      +15     
==========================================
+ Hits          395      410      +15     
  Misses        264      264

Impacted Files	Coverage Δ
src/AutoMerge/new-package.jl	75% <100%> (+1.08%)	⬆️
src/AutoMerge/new-version.jl	71.11% <100%> (+1.34%)	⬆️
src/AutoMerge/guidelines.jl	68.86% <100%> (+3.6%)	⬆️

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