[WIP] Speedup startup by beeing lazy.

[Carreau]

A significant chunk of the startup time is spent loading pygments
lexers.

This make it lazy which slightly improve the hot and cold startup time.

time ipython -c exit() cold goes from ~0.7 to ~0.65 seconds on my work
laptop I expect much more on old or slower machines.

[Carreau]

I realized that on my machine about 1/2 the startup time comes from the setuptools's installed ipython's script that loads pkg_resources, and then loads entry points.

@takluyver you are our installer expert; any idea ?

[takluyver]

Since a couple of years ago, pygments should lazy-load pkg_resources only if you try to list all lexers/formatters/styles/filters:
https://bitbucket.org/birkenfeld/pygments-main/pull-requests/690/delay-loading-pkg_resources-until-it-is/diff

If we can avoid triggering that kind of listing, we shouldn't need an extra lazy-loading layer on top of it.

As for the script: if you install with pip (and not the -e flag), you should get a script that doesn't use pkg_resources. pip creates the simpler scripts with distlib, rather than setuptools. For this reason, I tend to create my own developer installs by doing pip install ., then manually replacing the installed folder in site-packages with a symlink. Or I use Flit, of course. ;-)