When a template describes a build block without a source reference, the
build should be considered invalid as we won't have a CoreBuild produced
as a result of the need to have both.
In current versions of Packer, this will produce an error message
hinting that nothing will happen because of the lack of either build or
source block.
This commit takes the defined block, and points out to it as missing a
source block as being the reason why nothing is happening, making it
clearer what is required for an HCL2 build to be processed.
When a `packer build' is run on a config that does not define anything
to be built, the process would run normally, and only state that no
artifacts were produced after builds were run successfully.
This is misleading as we reach the end of the command without having a
proper warning that nothing was even attempted to be built.
This commit adds a warning when that happens, and leaves immediately
after.
