Move examples into mesa

[Corvince]

Based on previous discussion and to rectify some errors made in #2381

See commits for atomic changes:

1. move examples into mesa folder - the main purpose of this PR
2. remove the "examples" package which was a mistake of Importable examples #2381
3. Update paths
4. Add an empty __init__.py file to each basic example. While I find this a bit annoying, I think its actually the best way to make the examples truly portable, combined with relative imports. Each example becomes its own "package" in python speak. Also updated the imports so now you can do from mesa.examples import BoltzmannWealthModel instead of from mesa.examples.basic import BoltzmannWealthModel

[quaquel]

The move makes sense and should resolve the various issues we have been running into. Thanks!

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔴 +4.2% [+3.0%, +5.3%]	🔵 -0.3% [-0.5%, -0.2%]
BoltzmannWealth	large	🔵 +1.5% [+1.0%, +1.9%]	🔵 +0.8% [+0.6%, +1.1%]
Schelling	small	🔵 +0.0% [-0.2%, +0.3%]	🔵 -0.4% [-0.7%, -0.1%]
Schelling	large	🔵 +1.1% [+0.4%, +1.9%]	🔴 +6.3% [+5.3%, +7.5%]
WolfSheep	small	🔵 +0.3% [+0.1%, +0.5%]	🔵 -0.1% [-0.2%, +0.1%]
WolfSheep	large	🔵 +0.3% [-0.3%, +1.0%]	🔵 +0.4% [-1.5%, +2.0%]
BoidFlockers	small	🔵 -0.1% [-0.7%, +0.6%]	🔵 +0.7% [-0.1%, +1.3%]
BoidFlockers	large	🔵 +0.6% [-0.5%, +1.5%]	🔵 -0.2% [-0.8%, +0.3%]

[EwoutH]

Good stuff, thanks.

Git history is maintained remarkable well!

[Corvince]

Yes, luckily for git its just a rename operation, since git doesn't know about the concept of folders.