Refactor UI package

[mackenbach]

This started out as a fix for the build problems introduced in #274, but ended up as a refactor of the ui package.

The main changes:

• ui.IO interface changed from:

type IO interface {
	Stdin() io.Reader
	Stdout() io.Writer
	Prompts() (io.Reader, io.Writer, error)
	IsStdinPiped() bool
	IsStdoutPiped() bool
}

to:

type IO interface {
	// Input returns an io,Reader that reads input for the current process.
	// If the process's input is piped, this reads from the pipe otherwise it asks input from the user.
	Input() io.Reader
	// Output returns an io.Writer that writes output for the current process.
	// If the process's output is piped, this writes to the pipe otherwise it prints to the terminal.
	Output() io.Writer
	// Stdin returns the *os.File of the current process's stdin stream.
	Stdin() *os.File
	// Stdin returns the *os.File of the current process's stdout stream.
	Stdout() *os.File
	// Prompts returns an io.Reader and io.Writer that read and write directly to/from the terminal, even if the
	// input or output of the current process is piped.
	// If this is not supported, an error is returned.
	Prompts() (io.Reader, io.Writer, error)
	// ReadSecret reads a line of input from the terminal while hiding the entered characters.
	// Returns an error if secret input is not supported.
	ReadSecret() ([]byte, error)
	// IsInputPiped returns whether the current process's input is piped from another process.
	IsInputPiped() bool
	// IsOutputPiped returns whether the current process's output is piped to another process.
	IsOutputPiped() bool
}

This allows users of the interface to explicitly request the underlying stdin or stdout file when they need it (for example, when spawning a child process). Also the naming of IsInputPiped and IsOutputPiped better describes the interface than IsStdinPiped and IsStdoutPiped.

• The interface is actually implemented by different structs (standardIO, windowsIO and ttyIO) instead of a single struct with different fields set.

This replaces the changes from #274.

[jpcoenen]

Build verification is introduced in #278.

[jpcoenen]

I think I fixed most of the issues. Could use some manual testing to verify whether it actually works on all platforms.

[SimonBarendse]

I believe #274 was also introduced to fix a bug, there is a package (I believe in the standard library) that changes its behavior based on the passed Writer being os.Stdout. Before the refactor not os.Stdout itself was returned, but a wrapped version. The refactor changed this so that os.Stdout would be detected as stdout by the package.

This shows the importance of descriptive and complete PR messages.

@Marton6 Do you remember which bug and package this was about exactly?

[SimonBarendse]

It's not clear to me which changes are required to fix the build and which are just refactors. I hope the questions I asked below clear things up, if not, let's have a quick call.

[jpcoenen]

Removed the bug label because the PR that introduced this has been reverted. (#279)

Changeset itself is still relevant.

[jpcoenen]

Since this is not really useful on itself, I am merging it into #269.

[mackenbach]

@jpcoenen feel free to merge when you feel it's ready