cmd/cue: rename 'get go' to 'import go'

[cueckoo]

Originally opened by @myitcv in cuelang/cue#646

Splitting out a separate issue for the rename of get go to import go from cuelang/cue#621 (comment)

Other points to consider:

• whether cue import go should adopt go test-like behaviour of making progress where it can. For example, if you run cue import go ./... and one package in the set that is matched by ./... fails to type check, then we should generate definitions for those packages that do type check rather than failing the entire process
• cue get go has a --local flag. In a world of modules this flag is, I think, redundant. Because if you were to run cue import go on a main module package, it would be strange for it to be placed within the cue.mod/ hierarchy. It only makes sense for such definitions to be generated alongside the Go code from which they were generated. Fixes cmd/cue: path resolution issue for get go generated files #452
• cue import go should run with GOFLAGS=-mod=readonly to enforce that all dependencies are available
• Fix the redundant walking of target package directories for .cue files (happens as part of the walk of the directories containing CompiledGoFiles)
• Fix the fact that generated code should be placed in the cue.mod/gen hierarchy, not the cue.mod/pkg as it is today
• Consider supporting cue import go $pkg@$version. This allows people to depend on a package (or packages) outside the dependency graph of their main module.

[myitcv]

As part of the recent issue garden, we are focussing on non-feature requests. As such, I'm removing the milestone on this feature request. We will revisit feature requests in a later pass, at which point we will start to milestone and prioritise new features (in addition to those that we are already working on).