Run autoloading separately from composer install

[sophiewigmore]

Summary

#5
Resolves failures to autoload classes outside of the vendor directory, caused by the composer install looking for classes inside of the layer, instead of the working directory.

The solution is:

• run composer install with the --no-autoload flag in ALL cases
• perform typical symlinking from the layer to the working directory,
• and then run composer dump-autoload from the working directory to do the autoloading, where composer can find classes outside of vendor.
• added a test case/app like the sample given by OP to demonstrate that this bug has been resolved

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[sophiewigmore]

bump @thitch97 @joshuatcasey - can I get a review on this please?