Move the main logic of `generateTaskContext` and `findTaskNeeds` to the
`services` layer.
This is a part of #32751, since we need the git context and `needs` to
parse the concurrency expressions.
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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
