remove duplicates in imports.go generated by 'flogo create'

[yxuco]

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

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

Fixes: #

What is the current behavior? imports.go generated by 'flogo create' contains many duplicated lines of same package

What is the new behavior? This fix will generate a cleaner imports.go that includes each unique package only once.