[luci-config] Use gitiles.Location.parse in imports_ref
[luci-config] Use gitiles.Location.parse in imports_ref

parse_resolve raises an exception if ref is invalid.
But ref in the location is irrelevant in import_ref because we need only
host and project. Use simpler gitiles.Location.parse.

Change-Id: I22451a8a117caeaee009d84b11315dd6e3f0d10e
Reviewed-on: https://chromium-review.googlesource.com/1121203
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py