We were previously, erroneously, not applying replace directives from
the main package's module's go.mod file for non-'-m' installs. When
calling gobin with -m, it is expected that such replace directives are
not applied. But without -m, we effectively treat the main package's
module as the main module (except that we actually treat it as a
dependency, in order to capture the version information in the resulting
binary).

This commit fixes that discrepancy, and locks it in with a two tests,
that compare the presence or otherwise of the replace directive between
the non-'-m' and -m modes.

Adapt implementation and tests to cover Mac and Windows.

Addressing feedback from #49 that accidentally arrived after I merged.

As part of that, fix test where behaviour of go command has changed
between 1.11 and 1.12 series.

This also requires that we upgrade our dependency on
github.com/rogpeppe/go-internal to take advantage of go version
conditions.

Because of golang/go#29773, circular module
requirements create a problem with Go 1.11 and 1.12beta2 (at least).

The fix for this is scheduled for Go 1.13, but we need to add a
dependency from myitcv.io -> github.com/myitcv/gobin now.

Given that the dependency on myitcv.io is only a development-time tool
dependency, the easiest thing to do for now is to drop that requirement,
thereby breaking the cycle.