use DIRECTORY_SEPARATOR everywhere

[crtlib]

I know it's not life-threatening, but I think it's a bit more conventional

[coveralls]

Coverage remained the same when pulling 8ba04a6 on crtlib:master into 9f4ccb6 on yiisoft:master.

[cebe]

Makes code less readable. We had discussion about that before, only use DIRECTORY_SEPARATOR when value might appear elsewhere for example in a config value, not in include or require.

Some references:
a6c251d#commitcomment-3291800
607f3a5#commitcomment-3335365