move working dir/dirstate methods from localrepo to workingctx

color: labeled text should be passed to ui.write() as ui.labeled Some implementations of ui.label() (HTML versions in particular) must escape the provided text and then markup the text with their tags. When this marked up text is then passed to ui.write(), we must label the text as 'ui.labeled' so the implementation knows not to escape it a second time (exposing the initial markup). This required the addition of a 'ui.plain' label for text that is purposefully not marked up. I was a little pedantic here, passing even ' ' strings to ui.label() when it would be included with other labeled text in a ui.write() call. But it seemed appropriate to lean to the side of caution.

move discovery methods from localrepo into new discovery module