Update magefile/mage to v1.8.0

[andrewkroh]

Vendor github.com/magefile/mage version v1.8.0 (aedfce64c122eef47009b7f80c9771044753215d).

I updated the make mage target to detect the mage version and automatically upgrade

Upgrade benefits:

• mage will automatically detect changes in dependencies of magefile.go because it always recompiles the magefile.go on Go 1.11. This feature relies on the GOCACHE to mitigate the expense of recompiling each time. It’s a little slower but more reliable.
• We can start to DRY out the magefiles because this version has support for target imports. It’s a simple Go import statement annotated with // mage:import [namespace].

TODO:

• Test behavior when moving between branches. We’ll likely need to backport make logic to re-install mage if the version is wrong (e.g. downgrade automatically).
  • The only thing that's not backwards compatible with the new mage version is executing package on non-Linux. This is because the new mage version added -goos and -goarch flags for use with -compile and no longer honors the GOOS and GOARCH when pre-compiling a mage binary that targets Linux for use inside the container.

[ruflin]

Could we potentially backport this to 6.x as I expect most development will happen now in 6.x and master. So change of branch should become less of a problem?