stack_install: let user apply source patches

[aherrmann]

Is your feature request related to a problem? Please describe.
With Hazel some Hackage packages require small source patches to build under Bazel. E.g. to adjust file paths to embedded files or runtime data dependencies. See wai-app-static for example. The new stack_install does not allow users to apply source patches.

Describe the solution you'd like
stack_install should allow users to specify source patches. Ideally in a way similar to http_archive.

Describe alternatives you've considered
Users could fork the package repository and if stack_install allowed to override a package's source repository users could inject patches that way. For small patches this is usually too much overhead.

Additional context
Opening issue as suggested in #887 (comment).

[mboes]

A simple but powerful way to achieve this would be to add a vendored_packages attribute that lets users include source checkouts local to the repo, or in a http_archive. Users can patch that source as much as they want, or include a BUILD.bazel, which would override any default target definition.

[aherrmann]

vendored_packages sounds like a very good solution to this problem.

[hanshoglund]

Looks like this is covered by #1060. Shall we close?