Check for manifest file existence based on absolute paths

[jonabc]

This is a small fix to the manifest source that is caused from the path format of Licensed::Git.files.

That function returns the contents from git ls-files --full-name --recurse-submodules, which includes the relative path of files from the repository root. The updated check here validates that files exist on the local system because they may have been deleted locally but not deleted from the git index. The error occurs because this check is done from the context of the application's source path, and will only succeed when the application's source path is also the repository root.

e.g. if this command is run in a folder foo/bar, Git.files will return file paths foo/bar/file1.rb. This fails because there is no file at foo/bar/file.rb relative to the current working directory foo/bar.

The fix is to check for existence of absolute paths that are rooted at the detected git repository root.