Fix path in test bootstrap to include the base testcase.
Remove unused namespaces from AbstractInstaller.
Do only combine VERSION value once and use it.
Extract Swift Mailer injection into a new method injectSwiftMailer().

We want to keep the cyclomatic complexity of methods low and it was too
big for injectContaoCore().
Remove obsolete use.
Change usages of property io to inputOutput.
Fix phpDoc in Plugin.php
Merge branch 'develop' of github.com:contao-community-alliance/composer-plugin into develop

Conflicts:
	src/ContaoCommunityAlliance/Composer/Plugin/Plugin.php
Pass Plugin instance to installer.
New testcase GetNativePathTest.
New testcase UnprefixPathTest.
New testcase UnprefixPathTest.
Merge branch 'develop' of github.com:contao-community-alliance/composer-plugin into develop
New test to check composer is only removed if not a special branch.

See fix in: d1d521f88711705287afbdd85178991a13c7380e.