[TODO] [pending #8636] rename stdlib.nimble to std.nimble for consistency

[timotheecour]

this now generates:
CC: std.pure.cookies
instead of:
CC: stdlib.pure.cookies

likewise with generated files under nimcache

this is more consistent with the fact we import via std/ospaths instead of stdlib/ospaths

• TODO: see if I need to rename these registerCallback(c, "stdlib.core.macros." & astToStr(op), op Wrapper) to s/stdlib/std/
• rename other mentions of stdlib, eg: tests/stdlib/

[Araq]

This would break code and seems unnecessary.