Error if we're not the first buildpack

[schneems]

I had someone using the heroku/metrics buildpack which writes to .profile.d directory running before this buildpack. Because this buildpack deletes everything at the root, including things from other build packs, the change was not preserved even though it looked like it executed correctly.

I'm proposing if we detect that another buildpack has executed (i.e. if the .profile.d folder is not empty), that we abort and error with a helpful message:

[main 2446fe5] empty
Enumerating objects: 1, done.
Counting objects: 100% (1/1), done.
Writing objects: 100% (1/1), 185 bytes | 185.00 KiB/s, done.
Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Building on the Heroku-20 stack
remote: -----> Using buildpacks:
remote:        1. heroku/metrics
remote:        2. https://github.com/schneems/subdir-heroku-buildpack#schneems/don-t-delete-profile-d
remote: -----> HerokuRuntimeMetrics app detected
remote: -----> Setting up .profile.d to automatically run metrics agent...
remote: -----> Subdir buildpack app detected
remote: -----> Subdir buildpack in api
remote:        Another buildpack seems to have run
remote:        before the subdir buildpack. This
remote:        may cause unexpected problems.
remote:
remote:        Ensure '$ heroku buildpacks' lists
remote:        the subdir buildpack first and try
remote:        again.
remote:  !     Push rejected, failed to compile Subdir buildpack app.
remote:
remote:  !     Push failed
remote: Verifying deploy...
remote:
remote: !	Push rejected to still-harbor-23981.
$ heroku buildpacks
=== still-harbor-23981 Buildpack URLs
1. heroku/metrics
2. https://github.com/schneems/subdir-heroku-buildpack#schneems/don-t-delete-profile-d