Support exclude patterns

[karalabe]

This PR contains an implementation of exclude patterns, where the user may specify certain packages )or folders) to exclude from vendoring. For the rationale behind this please see #54.

The exclusion pattern in the plain config file format is an import path with an dash (-) preceding it, so it's unambiguous and backwards compatible with the current mode of operation. In yaml format a new exclude field is used which contains a list of paths to exclude. If an exclude pattern is present in the config file, trash will first delete those from any vendored packages and only then proceed with the removeUnusedImports step, ensuring that anything that only excluded packages depended on is also removed.

---

Beside the above feature, the PR contains two bugfixes:

Previously a trash --update overwrote any existing config file without retaining its original format (#46). This had the unfortunate side effect that an original yaml config file was swapped out with a flat conf type. A proposal to fix this was done in #50, but adding a new flag is a redundant operation since the code already knows what the original format was. This PR ensures that the original config file format (yaml or conf) is retained inside the Conf struct, allowing to serialize back to the same format on a dump.

Another bug was that trash recreated a new config structure from scratch on update, overwriting the old struct before the package metadata (version, external repo) could be extracted, losing all such information. A proposal was opened in #51 to fix this, but it doesn't play nice with other internal/private fields in the Conf struct. This PR offers an alternative solution by not recreating the original config struct, just its Imports field. This was all private metadata is preserved, including the importMap containing the import meta fields.

[imikushin]

Can you separate the bug fixes from #54 implementation? Let's keep things as unentangled as possible :)

Anyways, thanks a lot for using and contributing, @karalabe !

[karalabe]

Thing is, both the yaml fix and the exclusion patters rely on fixing the metadata bug, and fixing it in this particular way opposed to the other currently open PR. Hence why I did a combo pull. If you want I can split them up, but that will require multiple rounds of merges.

[karalabe]

Meh, here's the bugfix PR :) #56

[karalabe]

I've also updated this PR to have the previous bugfix commit and a separate commit for the exclusion patterns. Should allow merging both PRs without conflicts :)

[imikushin]

@karalabe I've had to rebase this PR's commit before I could merge it (my fault, sorry!). Good thing, the change is actually merged into master now.

I'm going to tag a new release shortly.

Thanks a ton for your contributions and patience! 🍺