why: easier to find since it's in node_modules, easy to clear all cache for all the tools that use it.

Move register cache file location to ./node_modules/.cache/babel-register via

https://github.com/avajs/find-cache-dir

Ref previous babel-loader PR: https://github.com/babel/babel-loader/pull/301/files as well as the current file (https://github.com/babel/babel-loader/blob/e9e37029e0c5905d8846f69d9a0c348f3d49fe50/src/fs-cache.js#L170), https://github.com/babel/babel-loader/pull/318/files

• Comment below you are going to do this (for others to know)
• Read CONTRIBUTING.md (make sure you have yarn, and are on at least node 4)
• Check you are on 7.0 branch with git checkout 7.0 and then make a branch off of that to PR
• remember to run make build after a source code change or better just run make watch in another terminal window to auto run babel while you are developing
• Default to findCacheDir({ name: "babel-register" }) and fallback to os/home/tmp dir.
• if there are already tests setup for that package, write another test (otherwise we'll do it or help)