Clear the cache when switching to a different package manager

[edmorley]

Currently if a build is performed using one package manager (eg Pipenv), and then a subsequent build is performed using a different package manager (eg pip), the build cache is not cleared, resulting in leftover files and packages (such as the Pipenv installation still being present).

This can be seen in these recently added tests:

heroku-buildpack-python/spec/hatchet/pip_spec.rb

Lines 104 to 126 in dba9b86

	context 'when the package manager has changed from Pipenv to pip since the last build' do
	let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_basic') }
	# TODO: Fix this case so the cache is actually cleared.
	it 'clears the cache before installing with pip' do
	app.deploy do |app|
	FileUtils.rm(['Pipfile', 'Pipfile.lock'])
	FileUtils.cp(FIXTURE_DIR.join('requirements_basic/.python-version'), '.')
	FileUtils.cp(FIXTURE_DIR.join('requirements_basic/requirements.txt'), '.')
	app.commit!
	app.push!
	expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
	remote: -----> Python app detected
	remote: -----> Using Python #{DEFAULT_PYTHON_MAJOR_VERSION} specified in .python-version
	remote: -----> Using cached install of Python #{DEFAULT_PYTHON_FULL_VERSION}
	remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
	remote: -----> Installing SQLite3
	remote: -----> Installing requirements with pip
	remote: -----> Discovering process types
	REGEX
	end
	end
	end

heroku-buildpack-python/spec/hatchet/pipenv_spec.rb

Lines 286 to 311 in dba9b86

	context 'when the package manager has changed from pip to Pipenv since the last build' do
	let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_basic') }
	# TODO: Fix this case so the cache is actually cleared.
	it 'clears the cache before installing with Pipenv' do
	app.deploy do |app|
	FileUtils.rm('.python-version')
	FileUtils.rm('requirements.txt')
	FileUtils.cp(FIXTURE_DIR.join('pipenv_basic/Pipfile'), '.')
	FileUtils.cp(FIXTURE_DIR.join('pipenv_basic/Pipfile.lock'), '.')
	app.commit!
	app.push!
	expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
	remote: -----> Python app detected
	remote: -----> Using Python #{DEFAULT_PYTHON_MAJOR_VERSION} specified in Pipfile.lock
	remote: -----> Using cached install of Python #{DEFAULT_PYTHON_FULL_VERSION}
	remote: -----> Installing pip #{PIP_VERSION}, setuptools #{SETUPTOOLS_VERSION} and wheel #{WHEEL_VERSION}
	remote: -----> Installing Pipenv #{PIPENV_VERSION}
	remote: -----> Installing SQLite3
	remote: -----> Installing dependencies with Pipenv
	remote: Installing dependencies from Pipfile.lock \\(.+\\)...
	remote: -----> Discovering process types
	REGEX
	end
	end
	end

Fixing this blocks adding support for Poetry in #796, since there will be a number of apps that may switch package manager to try out Poetry.