Tidy import of packages and remove unnecessary prefixes

[odow]

Our imports and prefixes were scattered throughout the source and often conflicted (both using LinearAlgebra and import LinearAlgebra, and some calls prefixed and some calls not).

[codecov]

Codecov Report

Base: 98.05% // Head: 98.06% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (bf070db) compared to base (e4b0610).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3186   +/-   ##
=======================================
  Coverage   98.05%   98.06%           
=======================================
  Files          33       33           
  Lines        4587     4591    +4     
=======================================
+ Hits         4498     4502    +4     
  Misses         89       89           

Impacted Files	Coverage Δ
src/Containers/Containers.jl	91.66% <ø> (ø)
src/Containers/macro.jl	100.00% <ø> (ø)
src/feasibility_checker.jl	100.00% <ø> (ø)
src/lp_sensitivity2.jl	97.76% <ø> (ø)
src/solution_summary.jl	97.50% <ø> (ø)
src/JuMP.jl	96.09% <100.00%> (ø)
src/aff_expr.jl	97.25% <100.00%> (ø)
src/constraints.jl	96.49% <100.00%> (ø)
src/copy.jl	96.15% <100.00%> (ø)
src/macros.jl	98.51% <100.00%> (ø)
... and 6 more

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