Clean up all build files

[xyb]

fix #555

[genotrance]

This is good work, can you please resolve conflicts and resubmit? Thank you.

Edit: Ideally, nimpretty changes could be done separately from the actual code changes so that it is easier to tell the difference. If possible, please squash into two commits - first with nimble clean related additions followed by a nimpretty commit.

[dom96]

Please separate into two PRs (one for nimpretty). Otherwise reviewing this is a pain.

[xyb]

@genotrance @dom96 Sure, I have separated first PR #774.

[xyb]

@dom96 @genotrance I removed changes created by nimpretty, now it's included nimble clean related only.

[timotheecour]

I think this is the wrong approach.

Here are some problems:

• if nimble test crashes for whatever reason (maybe even Control C), generated files/dirs won't get cleaned up; even on the next run; eg:
  let existsBefore = existsFile(binFileName) would be true on next run. This can be addressed, but at the risk of introducing false positives

• the algorithm to find files that should be cleaned up is fragile. Right now it just collects tfoo for a file tfoo.nim. This won't work if binaries are named differently, or for generated shared libraries or if tests generate other binary or non binary files, as is even the case with nimble's own nimble test, which generates other non binary files eg .json, .nimble, .nims extensions. This is not unique to nimble's tests, nim's compiler nimble pkg, and others.

• a tool cleaning up files using a complex logic makes me nervous it could also remove files that should not be removed, which would be even worse. The logic may not be complex right now but it will be if it intends to reduce those false negatives

• nimble test ideally should keep git status clean (even before nimble clean); this approach doesn't do that

Finally, a use case this prevents is to allow comparing 2 runs of nimble test and compare the output artifacts.

see #787 for what I'm proposing instead.