Inline imports redivivus

[Andrei Alexandrescu]

Mathias Lang just told me the bug preventing inline imports from working 
has been fixed, so I reopened this:

https://github.com/dlang/druntime/pull/1756

I think it's a very useful facility, more clearly so for large projects, 
and deserves a fair shake of the stick.

If it works well in practice, a future language proposal could take 
`_import!"std.datetime".SysTime` to the simpler and better 
`import(std.datetime).SysTime`. Using it as a library facility seems 
like a good step.