fx.Self: a parameter to fx.As for providing a type as itself

[JacobOaks]

We frequently see requests for folks who want to use fx.As to provide a type as another type, while also providing it as itself. To name a few:

• Prevent duplicate initialisation #1196
• How to provide a dependency as itself and as an interface implementation? #1148
• Annotate returning it self and Interfaces #1079

This is currently not possible via strictly using fx.As + a single constructor, since fx.As causes a constructor to no longer provide its original type. The workaround we often give is for folks to do something like this, which involves adding a second "constructor":

fx.Provide(
    newConcreteType,
    func(ct *concreteType) Interface {
        return ct
    }
)

which is admittedly not very ergonomic. A somewhat common pattern mistaken to be a workaround is providing the constructor twice instead:

fx.Provide(
    newConcreteType,
    fx.Annotate(newConcreteType, fx.As(new(Interface))),
)

This PR adds fx.Self(), which returns a special value to indicate an fx.As should retain that original return type:

fx.Provide(
    newConcreteType,
    fx.As(fx.Self()),
    fx.As(new(Interface)),
)

As an alternative, I considered a new annotation altogether, named something like fx.AlsoAs, but adding a special type that can be passed as an argument to fx.As directly allows for more fine-tuned control over individual positional return values.

For example, this function's return types can be easily expressed as *asStringer and io.Writer using fx.Self():

fx.Provide(
	fx.Annotate(
		func() (*asStringer, *bytes.Buffer) {/* ... */ },
		fx.As(fx.Self(), new(io.Writer)), // return values will be: *asStringer, io.Writer
	),
),

Whereas something like fx.AlsoAs wouldn't provide the ability to skip over the first positional return value entirely.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.46%. Comparing base (cb9cccf) to head (afe49a5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1201      +/-   ##
==========================================
- Coverage   98.49%   98.46%   -0.03%     
==========================================
  Files          34       34              
  Lines        2860     2676     -184     
==========================================
- Hits         2817     2635     -182     
+ Misses         36       34       -2     
  Partials        7        7              

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[tchung1118]

Changes look great and pretty isolated. Added a couple of comments.

[JacobOaks]

Changes look great and pretty isolated. Added a couple of comments.

Addressed!

[tchung1118]

LGTM