tidy extdeps: don't read whole lockfile into memory

[yotamofek]

also, remove outdated FIXME, and simplify bespoke toml parsing

Might improve tidy's memory consumption by a tiny bit 😅

[rustbot]

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[Kobzol]

I found the previous version to be easier to read/understand, tbh. And read_to_string should be fine here.

[yotamofek]

That's reasonable :)
So closing this. Thanks for taking a look!