Make will now delegate to mage for generating fields.yml. Make will check if the mage command exists and go install it if not. The FIELDS_FILE_PATH make variable is not longer used because the path(s) are specified in magefile.go.

This allows fields.yml to be generated on Windows using Mage. The CI scripts for Windows have been updated so that fields.yml is generated for all Beats during testing.

This also adds a make.bat in each directory where building occurs to give Windows
users a starting point.

Some fixes were made to the generators because:
- rsync was excluding important source files contained in a directory
  named "build"
- the generated project needed to be `git init` before running certain
  magefile targets that detect project's root dir and import path.