Update helm-source-projectile-buffers-list

Use idiomatic way to build helm-source-projectile-buffers-list. For this
source, we need to create our own specialized helm-source-projectile-buffer
class that combines both attributes in helm-source-in-buffer and
helm-type-buffer classes to make Helm displays this source using its
buffer UI.

We have to explicitly use helm-make-source rather than
helm-build-in-buffer-source because we use custom class.
helm-build-in-buffer-source is just a wrapper for helm-make-source that
builds source for the class helm-source-in-buffer.