Lazily load pygments styles

We currently try to retrieve all pygments styles at once, but this operation is very slow due to the import of pkg_resources on the pygments side. https://github.com/httpie/httpie/blob/cfcd7413d1afbabc5ab3c88a892851335be4944a/httpie/output/formatters/colors.py#L31

We should load these styles lazily when a user specifies --style explicitly.

[isidentical]

Command: hyperfine 'http --version'

	normal	lazy loading	factor
Environment 1	238.2 ms	185.4 ms	1.30X (30%)
Environment 2	383.3 ms	240.4 ms	1.58X (58%)

Since the actual load we are removing is pkg_resources import, the speed-up factor is very dependant on how complex the current python environment you are within. The Environment 1 is simply a fresh python docker image with only httpie installed. The Environment 2 is my local environment with over 300+ python packages.

[isidentical]

Couldn’t we use a different method for checking if it exists (e.g. checking if a module exists first, and only then loading the styles if not)?

We could introduce layers (e.g check if this is a built-in httpie style > check if this is a built-in pygments style > check if this is coming from a pygments plugin), but I think that would be a bit complicated.

Another possibility is asking pygments maintainers whether they would accept a patch that migrates from pkg_resources to importlib_metadata (a much much faster version for iterating over entry points), if the use case where a --style is also important to us.