pluto_export.json endpoint performance

[fonsp]

Requesting the /pluto_export.json has pretty bad preformance, it currently takes about 1s for the server to respond. Let's fix it!

[fonsp]

Might be caused by calling Pluto.frontmatter with a String here:

PlutoSliderServer.jl/src/IndexJSON.jl

Line 35 in 1418465

joinpath(start_dir, s.path)

which calls load_notebook_nobackup which will call

https://github.com/fonsp/Pluto.jl/blob/42fa2ce5fa560c676b2dd8280090dc35ad924a27/src/notebook/saving%20and%20loading.jl#L269-L281

which is a bunch of IO that we might want to avoid, which might make it related to #145

our profiling showed that a lot of time was spent in Pkg.write_env_usage

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A solution would be that Pluto.load_notebook_nobackup would not store the Pkg.EnvCache, but some proxy object. And the actual Pkg.EnvCache would only get created when needed.