Remove dep on DynamicPPL in package extentions

[sunxd3]

This PR remove further dependency of DynamicPPL in package extensions.

• in MCMCChains extension, varname_leaves util function is copied from DynamicPPL
• DynamicPPL extension used to have a single function that produce a DynamicPPL compatible SimpleVarInfo. This is still supported, only now the function is moved into the main package and produce a OrderDict{<:VarName, }, so when we make the integration, a single SimpleVarInfo(ordered_dict) would work.

[github-actions]

BridgeStan not found at location specified by $BRIDGESTAN environment variable, downloading version 2.5.0 to /home/runner/.bridgestan/bridgestan-2.5.0
Done!
Model dogs produces error: ErrorException("log_density() failed with exception: Exception: bernoulli_lpmf: Probability parameter is inf, but must be in the interval [0, 1] (in '/home/runner/work/JuliaBUGS.jl/JuliaBUGS.jl/benchmark/stan/bugs_examples/vol1/dogs/dogs.stan', line 37, column 6 to line 38, column 62)\n")

Model	Parameter Count	Data Count	Stan Density Time (µs)	Stan Density Gradient Time (µs)	JuliaBUGS Density Time with Graph Walk (µs)	JuliaBUGS Density Gradient Time with ReverseDiff.jl(compiled tape) (µs)
rats	65	150	5.6326	8.21867	6977.95	101.47
pumps	12	10	1.00462	1.39309	176.32	6.745
dogs	2	720	NA	NA	3550.27	151.353
seeds	26	21	2.47827	3.15583	701.39	20.839
surgical_realistic	14	12	1.24025	1.70317	236.653	9.01367
magnesium	108	96	10.6195	12.5385	9274.57	80.851
salm	22	18	2.58936	3.255	494.894	13.866
equiv	15	20	2.53482	3.45138	368.177	17.853
dyes	9	30	1.01475	1.38067	242.462	13.405
stacks	6	21	1.13904	1.74794	325.168	14.878
epil	303	236	33.012	39.714	64558.0	283.85
blockers	47	44	3.28389	3.76129	2020.17	31.159
oxford	244	240	16.14	18.916	48133.6	194.303
lsat	1006	5000	181.048	222.845	2.99378e6	3975.85
bones	33	422	72.766	92.943	14504.9	199.232
mice	20	65	7.33625	9.36733	1031.13	40.235
kidney	64	58	10.7	16.29	3698.82	76.413
leuk	18	714	20.437	26.81	9723.04	207.077
leukfr	40	714	24.045	31.809	18609.9	255.185

[coveralls]

Pull Request Test Coverage Report for Build 11487515514

Details

• 1 of 21 (4.76%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 77.678%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ext/JuliaBUGSMCMCChainsExt.jl	1	10	10.0%
src/model.jl	0	11	0.0%

Totals
Change from base Build 11433118017:	-0.3%
Covered Lines:	1378
Relevant Lines:	1774

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💛 - Coveralls