Improve core/latestposts block PHP code

[nylen]

Addresses https://github.com/WordPress/gutenberg/pull/870/files#r120894090 and https://github.com/WordPress/gutenberg/pull/870/files#r121213401:

• Removes unneeded PHP constants.
• Simplifies server-side block registration logic. Currently there should be no need to defer this until init, at least in our main plugin.
• Moves the new PHP code for this block under the lib/ directory so that it will be scanned by phpcs (also broken currently, another PR coming up for this).
• Adds the new PHP code to the plugin zip. This fixes npm run package-plugin - currently it generates broken builds.

Placing PHP code inline with its block code in the blocks/library directory makes these last two steps in particular a good bit more difficult. I think all PHP code should live under lib/ and all JavaScript code should live elsewhere (under the already-defined Webpack entry points); given that we have to do path manipulation every time we mix the two, it doesn't seem like a good idea.

[lamosty]

Thanks for doing this!

[nylen]

No problem. I created #1097 to discuss the server-side initialization further.

[westonruter]

@nylen I think the PHP logic for blocks should be placed in the same directory as the JS logic. I've proposed this in #2014. By keeping all of the related logic for a block in the same directory, it will be easier to locate the relevant files to edit when hacking.