client/isolate: factor-out isolate file parsing
client/isolate: factor-out isolate file parsing

Make available the isolate parsing logic so that other parts of the
codebase can successfully parse/expand isolate definitions.

Issue luci/luci-go#9

BUG=59899
R=mcgreevy@chromium.org, tansell@chromium.org

Review-Url: https://codereview.chromium.org/2500333002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go