Layout specification and discovery should follow the same behavior as partials

Fixes #1047
existing code but misleading:
- neither `nameRepresentsPath` nor `IsSpecificPath()` describes what this method does.  looks like it's close to `isApplicationRelativePath`
- use same name (modulo casing) for the method and the variable
- if you actually want to special-case any relative path, change the method to check all characters for '/'