Creates four subpackages for now:
1. kn/commands - inludes types.go for common
struct and other common files and misc commands
2. kn/commands/service - all 'kn service *' commands
3. kn/commands/revision - all 'kn revision *' commands
4. kn/core - contains the root.go and other top level
for code and testing
5. refactors:
a. split .../commons/human_readable_flags.go into three
b. modifies the HumanReadableFlags.ToPrinter to get pass
a function that sets the columns and fields
Had to refactor all tests to avoid cycles.
* Renames test names for service get command
also removes unnecessary commented imports
* Adds revision get command
renames `revision list` ==> `revision get`
* Adds tests for revision get command
* Updates the vendors/modules.txt
* Removes revision list command
* Adds service get command
replaces service list command
* Updates go.mod and vendors/*
* Adds message if no services found in requested namespace
* Adds tests for service get
dr.max