Don't mark sources as done as they might be dependencies elsewhere

[tiagovtristao]

This PR fixes #2077.

• Instead of marking sources as done (as they might be dependencies of other rules), do not include them in the list of dependencies that we iterate on. This slightly increases the time of preparing sources for targets where affected dependencies weren't previously being traversed/prepared.
• Fixes a bug where although sources where marked as done previously, requires/provides functionality was still taking place over marked sources.
• The logic is behind a FF as it might be disruptive.
• Fixes benchmarks around IterInputs where the real metrics are one order of magnitude higher than those previously recorded.

Before this PR:

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz
BenchmarkIterInputsControl
BenchmarkIterInputsControl-12             520227              2155 ns/op             688 B/op          7 allocs/op
BenchmarkIterInputsSimple
BenchmarkIterInputsSimple-12              175366              7727 ns/op            1584 B/op          8 allocs/op
BenchmarkIterInputsNamedSources
BenchmarkIterInputsNamedSources-12        129492             11883 ns/op            4192 B/op         20 allocs/o

Benchmarks fixed:

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz
BenchmarkIterInputsControl
BenchmarkIterInputsControl-12             470923              2217 ns/op             688 B/op          7 allocs/op
BenchmarkIterInputsSimple
BenchmarkIterInputsSimple-12               10000            106134 ns/op           33837 B/op        415 allocs/op
BenchmarkIterInputsNamedSources
BenchmarkIterInputsNamedSources-12         10000            100276 ns/op           36443 B/op        427 allocs/o

Benchmarks fixed and IterInputs fixed:

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) E-2136 CPU @ 3.30GHz
BenchmarkIterInputsControl
BenchmarkIterInputsControl-12             547437              2298 ns/op             688 B/op          7 allocs/op
BenchmarkIterInputsSimple
BenchmarkIterInputsSimple-12               13160             87224 ns/op           33835 B/op        415 allocs/op
BenchmarkIterInputsNamedSources
BenchmarkIterInputsNamedSources-12         10000            112870 ns/op           35142 B/op        421 allocs/op

[Tatskaari]

Looks like this change will increase the number of files in build directories in a few places. We need to add output_is_complete in a few places e.g. the go import config rules.

[Tatskaari]

Looks good to me :)