findpkg: unit tests

[hugelgupf]

pkg/findpkg needs some good test coverage of both Go module mode and GOPATH mode look ups of packages. Cases to consider:

• looking up directories that don't contain any Go files
• looking up packages in Go module mode
• looking up packages in GOPATH mode
• looking up Go modules outside of GOPATH in GOPATH mode
• looking up a Go package that isn't a command
• looking up a directory that only contains Go test files, but no Go command/package
• looking up more than one command, but one has a go.mod and the other does not, in Go module mode
• looking up 2 or more Go commands in 2 or more Go modules (testing the batching by module)

Due to #60 we need not consider looking up non-filesystem paths.

Rec: add directories with the desired scenario of module/package/command setups to the ./test and ./vendortest directories at the root of the repo to use in the test

[hugelgupf]

Solved by #79.