[FEA] Public header and detail header files should have the same names

[ttnghia]

As the title said, the public and detail headers should have the same names. Currently there are some inconsistencies between them. For example:

• Public header: cudf/copying.hpp,
• Detail header: cudf/detail/copy.hpp.

We should change the file names to make them consistent.

[davidwendt]

I'm not sure I agree with this global rule. I noticed that some detail functions are in separate files as needed. What makes sense to group together for the public API may not apply to internal detail functions used by multiple internal components.