add note to compile about legacy php support

TODO: replace with migration article, once written
Update detect
Drop composer.json from legacy branch, not needed
force php legacy mode
add custom build steps to legacy buildpack for jia purposes
Revert "add custom build steps to legacy buildpack for jia purposes"

This reverts commit 5beb5d35f236b5caee28d5d61f665bb0864a8bda.
merge legacy into master