Skip XDG base directories

[djc]

Consensus in today's meeting was that the ~/.xdg and /etc/xdg are not adding any value and are unidiomatic even in most Linux environments. Since we still use the directories crate, it will take XDG_CONFIG_HOME and XDG_CACHE_HOME into account for people who want to customize it.

[codspeed]

Merging this PR will improve performance by 15.31%

⚡ 1 improved benchmark
✅ 3 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	load-manifest	171 µs	148.3 µs	+15.31%

---

_{Comparing no-xdg (091465f) with main (ca79075)}

[djc]

I think we discussed this before, but I don't remember where: PACKAGING.md mentions a "system-wide configuration file at /etc/xdg/upki/config.toml" but I don't think that's currently what gets used? PACKAGING.md is the only one containing /etc and the platforms module is the only one referencing the directories crate, but it seems to exclusively build user-relative paths.

[ctz]

The binary package proposed in #61 used this. I think it's definitely a requirement that upki installed from a package works on a system-wide basis, but I don't have a strong view whether the global configuration file is at /etc/upki/config.toml or /etc/xdg/upki/config.toml or elsewhere.

[djc]

The binary package proposed in #61 used this. I think it's definitely a requirement that upki installed from a package works on a system-wide basis, but I don't have a strong view whether the global configuration file is at /etc/upki/config.toml or /etc/xdg/upki/config.toml or elsewhere.

I'd like to encode system-wide configuration paths and cache defaults in the library, probably after this is merged?