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

[myitcv]

Splitting out a separate issue for the rename of get go to import go from #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
• 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