There is no information on how the file in an import expression is located. The simplest would be to use the module load path.
The string import expressions are even more obscure. Current implementation doesn't even allow string importing files in a directory, this is not stated anywhere in the specs, but I think it should, otherwise is very hard to write portable code that uses string imports. I think the restriction should be removed too, see bug 3420.
Where the files are looked for is an implementation defined detail, and does not belong in the language specification. On the dmd page http://dlang.org/dmd-windows.html under the -J switch is the lookup detail.