Extract generate_index command to rubygems-generate_index gem #6

Job	Run time
Rubygems on Ubuntu (2.6)	21s
Rubygems on Ubuntu (2.7)	25s
Rubygems on Ubuntu (3.0)	9s
Rubygems on Ubuntu (3.1)	12s
Rubygems on Ubuntu (3.2)	10s
Rubygems on macOS (2.6)	48s
Rubygems on macOS (2.7)	48s
Rubygems on macOS (3.0)	16s
Rubygems on macOS (3.1)	29s
Rubygems on macOS (3.2)	18s
Rubygems on Windows (2.6)	5m 28s
Rubygems on Windows (2.7)	2m 35s
Rubygems on Windows (3.0)	2m 10s
Rubygems on Windows (3.1)	2m 25s
Rubygems on Windows (3.2)	3m 13s
Rubygems on Ubuntu (jruby-9.4)	51s
Rubygems on Ubuntu (truffleruby-22)	48s
Rubygems on Windows (mswin)	1m 43s
Rubygems on Ubuntu (ruby-head)	14s
Rubygems on Ubuntu (truffleruby-head)	35s
	23m 58s

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