Add composer-install buildpack

[joshuatcasey]

Summary

See README. This buildpack runs composer install to download and install Composer packages for PHP applications.

Resolves #4

Context

Per PHP RFC 0001 we are restructuring the PHP language family of buildpacks to make them more modular, up-to-date, and as a result, maintainable.

TODOs:

See TODO section in the README.

Issue

This issue is create the composer-install buildpack, but it is blocked on paketo-buildpacks/composer#12

• Rework it: This buildpack will be responsible for resolving, and installing project dependencies using composer only
• Rename: This buildpack (php-composer) should be renamed to (composer-install)
• API: The composer-install buildpack should provide none and require php and require composer at build-time.
• Packit: This buildpack should be written using the packit v2 library.
• Buildpack API Version: This buildpack should be written using Buildpack API 0.7.
• Add a [[buildpack.licences]] section to the buildpack.toml.
• README: Make sure a README is added that conveys context about the buildpack in a manner consistent with other Paketo buildpacks.
• Support env var configuration instead of buildpack.yml (See PHP buildpack RFC paketo-buildpacks/php-composer#2)

Acceptance

1. The buildpack provides none and requires php and requires composer
2. I can use this buildpack in conjunction with the PHP Dist buildpack and Composer buildpack to successfully build some composer app

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]

Once the linter is fixed, this is goooood to go!

[sophiewigmore]

Lint Workflows / lintYaml (pull_request) workflow is irrelevant, we can merge without because the regular lint job is fine!