Add support for Go modules

[nmiyake]

Use golang.org/x/tools/go/packages to load packages.
Fixes #34.

[nmiyake]

I believe the logic here is correct, although it's hard to tell if this maintains correctness in all cases due to the repo's lack of tests.

However, it does properly fix/address the basic case described in #34: after building with this fix, unconvert properly flags the error in the Go module:

➜  $GOPATH/src/github.com/mdempsky/unconvert/unconvert .
/Volumes/git/go-mod-playground/test-mod/main.go:1:85: unnecessary conversion

[nmiyake]

Oops, did not see #32 -- that change looks more comprehensive and was opened earlier, so closing this PR and will iterate/comment on that PR as needed.