Enhance yarn <argv> command to lookup global modules registry

accepted/0000-yarn-global-execute.md

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Yarn Issue: (leave this empty)
+
+# Summary
+
+Inspired by [`npx`](https://medium.com/@maybekatz/introducing-npx-an-npm-package-runner-55f7d4bd282b), enhance `yarn <argv>` command to lookup global modules registry and prompt to install if there's no such module or it's outdated.
+
+# Motivation
+
+There are packages (CLIs) that both serve as global and local binaries, examples are `react-native`, `jest`, `eslint`, `ember-cli` to name a few.
+With `npx` we can do:
+
+```sh
+npx react-native init MyApp
+# fetching react-native, populating npm local cache
+# running locally cached react-native init MyApp
+```
+
+But Yarn is not tied to Node foundation and it's not possible to introduce new binaries just like that but yarn-compatible.
+
+However, Yarn has this great feature of using `yarn <argv>` command to run npm scripts or look up local node_modules to execute binaries. Seems like a natural evolution to expand this command to be even more powerful and search for binaries even further.
+
+# Detailed design
+
+We could use existing `yarn create` command that does similar thing (populate global node_modules and execute the package with `create-` prefix, just without this prefix)
+
+When running `yarn package` the lookup would now look like this:
+
+- check for `package` in `package.json` scripts, execute (current)
+- check for `package` in local bin, execute (current)
+- check for `package` in global bin (new)
+  - __if__ latest version, print "running global module", execute, __else__
+  - prompt to download new version,
+    - __if__ confirm, execute, __else__
+    - exit
+
+## Example:
+
+When there's no `react-native` global binary:
+
+```sh
+yarn react-native init MyApp
+info: binary not found; use "react-native@0.59" from registry? (Y/n)
+# fetching react-native, populating yarn global cache
+# running global react-native init MyApp
+yarn jest
+yarn react-native run-ios # from local install
+```
+
+When latest version of binary is available:
+
+```sh
+yarn react-native init MyApp
+info: using global binary of "react-native@0.59"
+# running global react-native init MyApp
+```
+
+When binary is outdated
+
+```sh
+yarn react-native init MyApp
+info: outdated "react-native@0.59" found; use "react-native@0.60" from registry? (Y/n)
+# running global react-native init MyApp
+```
+
+# How We Teach This
+
+Extend the docs on default `yarn` command.
+
+# Drawbacks
+
+- increased complexity of `yarn` command
+- `yarn create` becomes obsolete (maybe an advantage though?)
+- not yet sure why, but I'd assume people may like to opt out from this behavior, could be put behind a config flag
+
+
+# Alternatives
+
+Put this behavior behind another command, like `yarn gx package-name`