This is a pre-requisite for making better use of the cache.

The current cache layout puts everything that the Node buildpack wants in the cache at: $CACHE_DIR/node including custom directories.

This creates a naming issue, since a user could cache anything. We currently keep a cache signature at: $CACHE_DIR/node/signature, but if a user declared a custom cache of a signature file, these would conflict and break things.

Adding anything else to the Node cache would make this problem worse, so let's fix it.

This adds one more level where all of the cached directories can live. Now if a user wants to cache a signature file, the cache layout would look like this:

node/
  - cache/
    - signature
  - signature
so there is no longer a conflict.

Deploying this will cause a cache miss for all users since the new code is looking in a new spot, but this is a common enough occurrence that I don't expect it to be significant for anyone. To ensure that the old cache will be discarded, I've added a version to the signature file.

This checks the user's package.json for a heroku-run-build-script key. If it is set to true the new build step will be run instead of the old step, and a message will be printed at the beginning of the build to confirm that is the case.

This adds a new logic branch that will run the 'build' script by default, and run the 'heroku-postbuild' script instead if it is defined.

This branch will not be run for any user yet. Further changes will start to enable this.

* Update default version to new Node LTS version v10.x

* Add Node 11 integration tests and metrics plugin

These tests use >= version requirements, so they would break whenever
new major versions are released. That's not really necessary, so this
removes the bits that will continually break.

Now that 1.11.0 is released with https://github.com/yarnpkg/yarn/pull/6413 merged,
Heroku users should see fewer failed builds due to 500 responses from
the registry.

The "Cached directories were not restored" message now includes the stack version in the list of possible reasons.

* Adjust output() so it preserves leading whitespace

Unless `IFS` is adjusted, `read` strips leading and trailing
whitespace from each line. In addition, without the `-r` argument
to `read`, unescaped backslashes are discarded.

See:
http://mywiki.wooledge.org/BashFAQ/001

Fixes #571.

* Add utility to monitor memory usage of subcommands

Update to v3 of the node plugin

We've seen an increasing failures due to frontend build exceeding Node
default 1.5GB memory limit during build. Heroku runs these builds on
machines with 2.5GB of memory. This increases the default limit for
Node 8 and above by passing commands via `NODE_OPTIONS` during build
if the user has not set this manually

* Rework output when caching directories

This splits the logic when the buildpack saves and restores the cache
into two separate branches:

- when the user hasn't specified specific directories to cache (most everyone)
- when the user has given a list of specific directories to cache (power users)

This gives us more control over the default messaging, which has
included messaging for bower_components forever, even though most users
will never use bower, nor should we encourage them to.

These changes silence and minimize the messaging for the default case

These changes move the hatchet ci specs from specs/ to specs/ci/ and creates a separate directory with some new specs that mimic the ones for CI, but are designed so that you can specify the Node version via ENV var so that they can be called from nodebin to test new Node releases before making them public

Adds hatchet tests as part of our testing matrix and adds that to Travis CI

The resolves an issue where a user that is not using a version of Node supported by the metrics plugin (not 8, 9, or 10) opts in to the feature.

Since the plugin for the appropriate version of Node is copied into the slug at build time, but the metrics url is set at runtime, we need to verify that the plugin exists before including it.

* Add metrics plugin + unit tests

This merges in the build changes and plugin from https://github.com/heroku/heroku-nodejs-metrics-buildpack which installs the metrics plugin https://github.com/heroku/heroku-nodejs-plugin at runtime once a Heroku use opts in to the language metrics feature

* Fix travis builds

- Yarn reworked the structure of their cache directory with a recent release, which broke one of the tests.

- added new fixtures for Node versions: 6, 8, 9, and 10

- Added new utilities for asserting that a file exists or does not exist after a build

Add buildpack.toml and publish script

Add heroku-18 to tests

Add more error detection for errors involving stack traces

Add detection for module 404s