Convert plugin bake to "copy" a folder

Expect a folder "Plugin" in a bake theme - and render each file found in
it recusrively. Generate everything in this way, even empty files, so
that users can define a plugin template in a theme without having any
hardcoded files forced upon them by the plugin task itself.
Test fixes for the plugin task
add license blocks to plugin template files
Add expected output when baking a simple plugin
Further test fixes

The test fails are in part caused by assumptions related to where things
are.

Changed the way the root composer file is found to be based on the ROOT
constant rather than "the folder above the plugins dir".
Changed all the content-comparison tests to be based on the
string-compare trait.
Attempt to clear up the actual intent of the test methods, by
simplifying and renaming test methods.
Code standard fixes
Fix whietspace in phpunit.xml.dist template
Update comments and variable names
we don't need composer updates on phpcs runs
Remove stray empty line
Correct closure cs error
rename env  vars to be more descriptive

"DEFAULT" doesn't really mean much - for example. Disassociate
installing dependencies from calling phpunit - other options need
dependencies installed too.
Rename again

to be consistent RUN_<this>
Merge pull request #6 from cakephp/better-plugin-bake

Better plugin bake