Create the previous dep graph index on a background thread

[Zoxc]

This changes SerializedDepGraph.index to be computed on-demand per dep kind. This means we can immediately start using queries without waiting for the entire index to be constructed. Additionally a background thread is started which computes the entire index, effectively off-loading most of the index construction to the background thread.

Benchmark	Before	After		Before	After
	Time	Time	%	Memory	Memory	%
🟣 clap:check:unchanged	0.4259s	0.4225s	-0.79%	89.65 MiB	90.08 MiB	0.48%
🟣 hyper:check:unchanged	0.1425s	0.1417s	-0.53%	47.85 MiB	47.91 MiB	0.13%
🟣 regex:check:unchanged	0.3188s	0.3157s	-0.97%	71.09 MiB	71.58 MiB	0.69%
🟣 syn:check:unchanged	0.5895s	0.5813s	💚 -1.38%	101.68 MiB	102.15 MiB	0.47%
🟣 syntex_syntax:check:unchanged	1.4392s	1.4361s	-0.22%	200.62 MiB	201.68 MiB	0.53%
Total	2.9158s	2.8974s	-0.63%	510.89 MiB	513.40 MiB	0.49%
Summary	1.0000s	0.9922s	-0.78%	1 byte	1.00 bytes	0.46%

Benchmark	Before	After		Before	After
	Time	Time	%	Memory	Memory	%
🟠 clap:debug:unchanged	1.0753s	1.0684s	-0.64%	142.80 MiB	142.72 MiB	-0.05%
🟠 hyper:debug:unchanged	0.2857s	0.2847s	-0.35%	63.06 MiB	63.15 MiB	0.13%
🟠 regex:debug:unchanged	0.7703s	0.7633s	-0.90%	108.76 MiB	109.03 MiB	0.25%
🟠 syn:debug:unchanged	1.0596s	1.0531s	-0.62%	142.08 MiB	142.18 MiB	0.07%
🟠 syntex_syntax:debug:unchanged	2.7530s	2.7274s	-0.93%	308.92 MiB	308.63 MiB	-0.09%
Total	5.9438s	5.8969s	-0.79%	765.62 MiB	765.71 MiB	0.01%
Summary	1.0000s	0.9931s	-0.69%	1 byte	1.00 bytes	0.06%

r? @cjgillot