perf(gen): goimports takes up to 60% time of generation

[tdakkota]

The CPU profile gathered from corpus collector latest run.

(pprof) top -cum x/tools
Active filters:
   focus=x/tools
Showing nodes accounting for 0.19s, 0.0093% of 2037.73s total
Dropped 977 nodes (cum <= 10.19s)
Showing top 10 nodes out of 226
      flat  flat%   sum%        cum   cum%
     0.04s 0.002% 0.002%    1162.91s 57.07%  golang.org/x/tools/internal/imports.Process
     0.04s 0.002% 0.0039%   1160.32s 56.94%  golang.org/x/tools/imports.Process
         0     0% 0.0039%   1157.15s 56.79%  github.com/ogen-go/ogen/gen.(*writer).Generate
         0     0% 0.0039%   1152.79s 56.57%  github.com/ogen-go/ogen/gen.(*Generator).WriteSource
         0     0% 0.0039%   1148.14s 56.34%  main.generate
         0     0% 0.0039%   1142.38s 56.06%  main.worker
         0     0% 0.0039%   1136.30s 55.76%  main.run.func7
         0     0% 0.0039%   1129.19s 55.41%  golang.org/x/sync/errgroup.(*Group).Go.func1

Also see: #66.