fix #14179, fix #14142, make CI 1.4x faster (2x faster locally)

[timotheecour]

• make CI significantly faster (1.4x faster on CI machines, 2x locally)
  by avoiding import os (and all the modules that it imports recursively) for every single invocation of nim; like wise for short-lived commands (eg nim dump or nim c smallfile or inim)

• fix nim dump is roughly 100x slower in 1.3 versus 1.2 #14179

• fix import os + use of existsDir/dirExists/existsFile/fileExists/findExe in config.nims causes "ambiguous call' error #14142

• supersedes [superseded] fix #14142: no more clash with: import os + use of existsDir/dirExists/existsFile/fileExists/findExe in config.nims #14143

• still keeps import os + use of paramCount and paramStr in config.nims causes "ambiguous call' error #12835 as fixed

see #14179 (comment) for the full context

CI timings on CI machines

before PR => after PR
Linux_amd64 31m => 21m
Linux_i386 32m => 18m
OSX_amd64 17m => 13m
OSX_amd64_cpp 18m => 13m
Windows_amd64 38m => 33m

CI timings locally on OSX

via XDG_CONFIG_HOME= nim c -r testament/testament all
494s => 253s
(didn't try ./koch runCI locally in this experiment)

[timotheecour]

@Araq PTAL, the last commit should address all your concerns (and also provide less misleading error msgs with nim js).

[Araq]

Thank you, superb work now.