Don't let goimports guess import paths

[vektah]

gqlgen internally tracks all the imports used in a file and adds them to the import block, handling duplicate imports by appending names.

There are two sources of imports:

• imports for user generated models, types in graphql, etc. These are all tracked as they are added, with the required deduping behaviours etc.
• ambientImports: these are referenced in the templates and must keep the name they were given in the templates, eg fmt.Println( must get the import name fmt. Often these are in conditional parts of the templates and may not be used in a particular generated output.

Currently we run the generated code through goimports, this does a few things for us:

• format the code: same as gofmt
• group and sort imports: this makes our output a little more robust to changes
• remove unused imports: required for ambient imports that don't actually get used.
• add missing imports: DANGER This adds in any missing imports, these often end up being ambient imports that were forgotten about. This leads to subtle and hard to reproduce issues like Ambiguous package names lead to incorrect import statements #207 when a user imported package grabs the name used in by the import in the template.

This PR removes the "add missing imports" feature of goimports, while maintaining the other 3 by reimplementing the import removal logic and calling goimports with "Format only" to get grouping and sorting.

Fixes #207

[jufemaiz]

QQ on this: is there a way of implementing local-prefixes option for goimports?