Implement `--{enable,disable}-executables`

This may have been subsumed by other tickets (e.g. #4087) but I figured it may be worth sketching a more modest enhancement which "completes" cabal's UI (scroll down for rationale):

## What?

We already have the ability to enable/disable tests and benchmark, via

```
--enable-tests Enable dependency checking and compilation
for test suites listed in the package
description file.
--disable-tests Disable dependency checking and
compilation for test suites listed in the
package description file.

--enable-benchmarks Enable dependency checking and compilation
for benchmarks listed in the package
description file.
--disable-benchmarks Disable dependency checking and
compilation for benchmarks listed in the
package description file.
```

which translates to `cabal.project` files as

```
tests: True
benchmarks: False

package bar
tests: False
benchmarks: True
```

as well as the [`--constraint` syntax](http://cabal.readthedocs.io/en/latest/installing-packages.html#cmdoption-setup-configure-constraint) to force the solver to enable tests/benchmarks

```
--constraint="bar bench"
--constraint="bar test"
```

(but I'm not sure this works properly in new-build btw; also I'm missing a `no-bench`/`no-test` token for symmetry)

What I'm suggesting now is to extend this logic to executable components, i.e.

```
--enable-executables Enable dependency checking and compilation
for executables listed in the package
description file.
--disable-executables Disable dependency checking and
compilation for executables listed in the
package description file.
```

and

```
--constraint="bar exe"
--constraint="bar no-exe"
```

as well as the respective `executables: ` property for `cabal.project` files

The default settings would be:

- executables are enabled by default for local packages (i.e. those specified via `packages: ...`)
- executables are obviously enabled by default where they're needed to satisfy build-tool-depends goals
- else, executables are disabled by default (e.g. when only the library of a package is requested via `build-depends`

## Why?

Finally, the rationale for this feature request (beyond improving symmetry of executables w/ tests & benchmark) is the situation that you have packages which bundle libraries + executables to form the unit of distribution. This has become more attractive with `new-build`'s per-component building, where you can depend on the library part of a package w/o cabal building all the executables (& their deps) as well.

However, cabal currently still has to solve for all executables even if all you ever need is the library component.

The library may often have a smaller dependency footprint and would therefore puts less pressure on the constraint solver.

However, the executables often have additional library dependencies, like e.g. `optparse-applicative` or even heavier dependencies (http libraries, GUI toolkits, terminal UI toolkits, etc).

So having to satisfy the executable's constraints provides no benefit for `build-depends` dependencies, and can instead result in the cabal solver not finding a solution, even though there would be if executables' constraints would have been ignored.