--importmain:foo to only cause import in main module

[timotheecour]

refs nim-lang#4451 (comment)
a flag --profiler should be sufficient and not require changing the sources with

when compileOption("profiler"):
  import nimprof

note that we can do --import:nimprof but that's also not satisfying as adds an import in all modules.

The exact same situation happens with libbacktrace, currently requiring either editing sources with import libbacktrace or passing --import:libbacktrace; see status-im/nim-libbacktrace#12

how about:

• adding --importmain:foo flag to nim which only does the automatic import for the main module being compiled
• making --profiler imply --importmain:profiler`
• making -d:nimStackTraceOverride imply --importmain:libbacktrace -g` (refs --import:libbacktrace, -g and --stacktrace:off shouldn't be needed status-im/nim-libbacktrace#12)

[timotheecour]

see also nim-lang#18251