Support classic and alternate import syntax in flogo.json

[debovema]

What kind of change does this PR introduce? (check one with "x")

[] Bugfix
[x] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

This PR is required to be merged before project-flogo/cli#32

What is the current behavior?
Currently imports in the "imports" array of flogo.json file follow the Go import path syntax (including aliases).
Hence it is not possible to know what is the path to the underlying module (if it exists) neither to specify a custom version.
This complicates the interaction of the Flogo CLI with go mod commands for instance in the flogo create command and it prevents the flogo.json to recreate an application in a reproducible way (since all declared imports will always used the @latest stable version available).

What is the new behavior?
In PR project-flogo/cli#32, a proposal is made to create a new syntax which would be more verbose while remaining readable.

This syntax is aimed at keeping good default values (no version = latest, no alias by default).
The only mandatory add-on is the : when the original import path does not point to a Go module like the github.com/project-flogo/contrib/activity/log contribution.

So

    "github.com/project-flogo/contrib/activity/log",

becomes

    "github.com/project-flogo/contrib:/activity/log",

Notice the : separating the path to the module and the relative import path.

Notice also that the github.com/project-flogo/flow import is unchanged as it already points to a Go module and it is expected to use latest version.

Update : this new syntax format can totally be mixed with the old one. Using the new one gives more flexibility to manage dependencies under the hood. Hence I suggest to name those two compatible syntaxes the classic syntax and the alternate syntax.

[debovema]

I updated the names of the syntaxes: the classic syntax and the alternate syntax

@fm-tibco : I reverted the examples/engine/flogo.json and created examples/alt/engine/flogo.json instead. So the default behaviour is to use the classic syntax. This way the alternate syntax will only be used by advised users.