Add composer source for PHP

[jonabc]

Closes #152

Adds a new dependency source enumerator for composer to enumerate php package dependencies.

The primary source of information is the composer.lock file. Since licensed requires dependencies to be installed locally before it can be run and php composer.phar install will autocreate a lockfile, it should be rare to have dependencies installed without a lockfile being present.

In order for dependencies to be enumerated

1. a php runtime needs to be present
2. a composer.lock file must be available at the licensed app source path
3. a composer application must be available

The composer application path defaults to composer.phar, which is appended to the licensed app source path for a final result of <repository root>/composer.phar. The composer application path can be overridden by adding a composer.application_path configuration value to .licensed.yml. The configuration value can be relative to the licensed app source path or an absolute path, and it can contain path directives like ~ or ...

BE WARNED
I haven't used PHP in about 15 years, and I haven't ever used composer before this. The implementation seemed relatively straightforward but if there are edge cases that aren't handled I will be asking those that submit issues for help 😆