PUP-2040 Add .pmtignore functionality

Prior to this PR there was no way for users building modules to ignore any
build artifacts of their own when running `puppet module build`.
This PR adds support for a `.pmtignore` file. The `.pmtignore` file contains
file-glob style patterns, one per line, to ignore from the module build process.
`Pathname.new(path).expand_path`?