Reflect in Current Directory First

[minicuts]

Fixes #354

Description
Reflect in Current Directory First

Some users have intention to run mockgen in a module aware directory with
version pinned to those in go.mod.

Always try to run reflect program in current working directory first.

More context about the root cause of the bug is in #354 (comment)

Submitter Checklist

• Includes tests

Reviewer Notes

• The code flow looks good.
• Tests added.

Release Notes

[cvgw]

So previously this code tried to execute reflect in

1. The root directory of the pkg in question
2. The current directory that mockgen was execute from
3. The tmp directory

This change would make it

1. The current directory that mockgen was execute from
2. The root directory of the pkg in question
3. The tmp directory

Is that correct?

[minicuts]

@cvgw exactly. To be more precise the execution in pkg happenes only with module support. Without module support mockgen would try to first execute in GOPATH.