Fix upreferred issue by loading at runtime

[MilesCranmer]

This fixes #12.

@j-fu what do you think? This seems to fix things for me:

julia> using Unitful

julia> using DynamicQuantities

julia> Unitful.preferunits(u"km")

julia> x_km = 1.5u"km"
1.5 km

julia> x = 1.5u"m"
1.5 m

julia> x_km_dyn = convert(DynamicQuantities.Quantity, x_km)
1500.0 𝐋 ¹

julia> x_dyn = convert(DynamicQuantities.Quantity, x)
1.5 𝐋 ¹

[github-actions]

Benchmark Results

	main	2e82902...	t[main]/t[2e82902...]
creation/Quantity(x)	4 ± 0.1 ns	5.3 ± 0.1 ns	0.755
creation/Quantity(x, length=y)	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
time_to_load	0.139 ± 0.0015 s	0.139 ± 0.0014 s	1
with_numbers/*real	3.3 ± 0.1 ns	3.3 ± 0.1 ns	1
with_numbers/^int	28.3 ± 2 ns	28.3 ± 1.9 ns	1
with_numbers/^int * real	28.5 ± 2.5 ns	28.5 ± 2.4 ns	1
with_quantity//y	17.8 ± 0.4 ns	17.8 ± 0.4 ns	1
with_quantity/^y	1.1 ± 0.2 μs	1.11 ± 0.2 μs	0.995
with_self/dimension	1.6 ± 0.1 ns	1.6 ± 0.1 ns	1
with_self/inv	18.5 ± 0 ns	18.5 ± 0 ns	1
with_self/ustrip	1.7 ± 0.1 ns	1.6 ± 0.1 ns	1.06

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).