Proc macro server: Load `ProcMacroHost` #7389

Job	Run time
build test linux x86-64	3m 40s
check-rust	1m 58s
build test windows x86-64	13m 12s
check-website	9s
scarb-metadata compatibility check	44s
doc tests	3m 10s
test linux x86-64 1/8	16m 17s
test linux x86-64 2/8	16m 59s
test linux x86-64 3/8	15m 33s
test linux x86-64 4/8	12m 52s
test linux x86-64 5/8	12m 36s
test linux x86-64 6/8	9m 27s
test linux x86-64 7/8	9m 22s
test linux x86-64 8/8	10m 28s
test windows x86-64 1/8	13m 38s
test windows x86-64 2/8	16m 36s
test windows x86-64 3/8	14m 1s
test windows x86-64 4/8	12m 27s
test windows x86-64 5/8	11m 24s
test windows x86-64 6/8	9m 22s
test windows x86-64 7/8	8m 35s
test windows x86-64 8/8	10m 14s
	3h 42m 44s

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