commands/operator-sdk/cmd: set composed GOPATH for Go projects (fix #712)

[estroz]

Description of the change: see #712 for main change. Also changed CheckAndGetCurrPkg() to CheckAndGetProjectGoPkg(), and removed local constants in favor of projutil contstants.

Motivation for the change: see #712, and make function name more obvious. We had to revert #712 in #721 so Ansible operator users don't see errors when they shouldn't. Now the GOPATH check only occurs when calling CheckAndGetProjectGoPkg().

@nrobert13

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In response to this:

Description of the change: see #712 for main change. Also changed CheckAndGetCurrPkg() to CheckAndGetProjectGoPkg(), and removed local constants in favor of projutil contstants.

Motivation for the change: see #712, and make function name more obvious. We had to revert #712 in #721 so Ansible operator users don't see errors when they shouldn't. Now the GOPATH check only occurs when calling CheckAndGetProjectGoPkg().

/cc @nrobert13

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[estroz]

One thing I'd like to note is that we require go 1.10+ be installed, so all users (Ansible or not) should have a GOPATH env variable set. However the only command requiring go is operator-sdk test local; otherwise there are checks in place to make sure certain sub-commands are being run in a Golang project. Should we make a note that this requirement is only for building/developing operator-sdk and development of Golang operators?

[hasbro17]

Should we make a note that this requirement is only for building/developing operator-sdk and development of Golang operators?

@estroz That's already mentioned in the ansible user-guide.
https://github.com/operator-framework/operator-sdk/blob/master/doc/ansible/user-guide.md#prerequisites

Of course they'll need it for the hybrid operator but that's another story.

[shawn-hurley]

LGTM