Add a built-in build_stage for applying patches #979
Description
Many packages have a build_stage for applying patches (typically before configure). This is common-enough that it's practically boilerplate, but for packages with a lot of patches it's also quite a bit tedious (see for example python.yaml).
Since this is such boilerplate it would be nice to have a built-in build_stage (like configure) that does this. Furthermore, while I can appreciate explicitness, it might be nice, in this case, to support some wildcards in filenames when supplying a list of patches to apply.
I only recently started looking seriously at hashdist though, so I don't know if this has been discussed before and if there's a reason not to do this. Otherwise I'm happy to supply a PR.