conform imports to schema defined in CONTRIBUTING.md

[laser]

Why does this PR exist?

Our imports don't match what's in the style guide:

### Conventions and Style

#### Imports
We use the following import ordering.

import (
        [stdlib packages, alpha-sorted]
        <single, empty line>
        [external packages]
        <single, empty line>
        [go-fil-markets packages]
)

What's in this PR?

This PR replaces the single-line imports with import-groupings, and clustered go-fil-markets imports as per CONTRIBUTING.md.

How do I achieve this awesome style myself?

On an OSX box, do this:

find . -type f -name \*.go | xargs -I '{}' goimports -w -local "github.com/filecoin-project/go-fil-markets" '{}'

then do this:

find . -type f -name \*.go | xargs -I '{}' sed -i.bak -e '/import (/ {
  :1
  $!N
  s/\n\n/\'$'\n''/
  /)/!b1
}' '{}'

then do this:

cd extern/filecoin-ffi ; git checkout . ; git clean -fd ; cd ../.. ; git checkout . ; git clean -fd

and finally:

find . -type f -name \*.go | xargs -I '{}' goimports -w -local "github.com/filecoin-project/go-fil-markets" '{}'

[hannahhoward]

In the original styleguide for go-filecoin, everything was all one filecoin repo.

I have been working under the assumption that if the repo is in filecoin-project, it should be in the last group. That’s why you may find many non-fil-markets deps in the bottom block. I realize the styleguide as written does not make that clear.

I guess I have no objection to changing it. LGTM unless you think it's worthy of further conversation.

[laser]

@hannahhoward

Makes sense. Would you mind approving? I will merge.

[hannahhoward]

sorry I thought I had