Stop PathForModuleSrc from validating the paths unnecessarily

PathForModuleSrc calls validatePath in order to convert the supplied
path components into a single path. Unfortunately, that corrupts a
fully qualified module name. So, when given "//namespace:module" it
treats it as a path and replaces "//" with "/". That replacement is
done by a call to filepath.Join().

This change simply concatenates the path components together textually,
using the path separator, to avoid the corruption. That ensures that
a fully qualified module name is preserved and processed properly. If
the path components do not contain a module name then expandOneSrcPath
will call pathForModuleSrc which validates the path so it does not open
up a way to create an invalid path as the validation was unnecessary
anyway.

Bug: 193228441
Test: m nothing
Change-Id: I0bb66feac182b77ce96c8d5d5f17e28ea28d75ba