`--genscript` messes up nimcache and future nim invocations

[timotheecour]

--genscript messes up nimcache because it copies over Nim/lib/nimbase.h (which could be stale, or from an old commit, or with local modifications etc) to nimcache/nimbase.h and then future invocations of nim (that don't even involve --genscript) will be forced to pickup up that old nimcache/nimbase.h instead of the current Nim/lib/nimbase.h; even nim c --forceBuild main.nim won't help with that

Example

git checkout commit_with_older_nimbase # or simply do a local temporary modification to nimbase
nim c --genscript main.nim
git checkout -

nim c main.nim #  `--forceBuild` doesn't help
uses old nimbase/nimbase.h

Possible Solution

• generate #include <nimbase.h> instead of #include "nimbase.h" in generateHeaders
  by using angle bracket include, it'll honor the -I flags and pickup the correct nimbase. However this may break the script generated by --genscript, which should be updated to add an appropriate -I flag

• nimbase.h should be tracked as a depdendency in the main.json file generated by --genscript and maybe also the generated main.deps file

Additional Information

• code where nimbase is copied for --genscript :

    copyFile(conf.libpath / RelativeFile"nimbase.h",
             getNimcacheDir(conf) / RelativeFile"nimbase.h")

• recent devel a33b72a
• root cause of one of the nimterop build errors I was having here build errors nimterop/nimterop#168 (comment)