lib/types: Fix path type check

Previously when this function was called without a value coercible to a
string it would throw an error instead of returning false. Now it does.

As a result this now allows the use of a type like `either path attrs`
without it erroring out when a definition is an attribute set.

The warning about there not being a isPath primop was removed because
this is not the case anymore, there is builtins.isPath. But also there
always was `builtins.typeOf x == "path"` that could've been used
instead. However the path type now stands for more than just path types,
but absolute paths in general.