Use search path for simple texture expressions

The path to the texture can be in any subdirectory of the root path
or within any subdirectory of the search path. The relative path to
the texture will be discovered automatically.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
As far as I know Boost does support relative paths.

In any case, I'm fine if you keep this function here for now. But I think it would make sense to migrate this to appleseed.foundation eventually.