Add a possibility to show types as their alias, rather than their underlying resolution, in mypy's output

[Fulguritude]

First of all, I'm sorry if this already exists and I'm just unaware of it.

Feature

Add some command-line argument to mypy to prevent the expansion/resolution of aliases of types (such as large type Unions; large chains of Literals; or heavily nested aliases) to their underlying type.

Pitch

Often, when running mypy, a type alias is "resolved" to its underlying type, which can sometimes severely impede legibility. This tends to happen a lot with Unions and Literals, as well as general chains of aliases.

Say I have an AlphabetDuos enum containing the 26^2 values "AA", "AB", ..., "ZY", "ZZ", and an accompanying literals list (with an alias) Literals_AlphabetDuos = Literal["AA", ..., "ZZ"] (because I need to serialize/deserialize those cleanly, for example). I've typed my input with Literals_AlphabetDuos, but MyPy will often correct you with a massive value of type 'str' cannot be assigned to type Literal["AA", ..., "ZZ"], expanding the 26^2 values and making the terminal output quickly illegible; when it could much more cleanly answer you with value of type 'str' cannot be assigned to type Literals_AlphabetDuos.

A similar issue happens with large Unions, and nesting of types. It's also easy to see how this automatic resolution gets unwieldy when you have things like Alias3 = Dict[AliasB, AliasD] throughout your code, and AliasB and AliasD are large literals or unions etc. Finding the right comma between parentheses or brackets (in a non-prettified string) is an excruciatingly annoying endeavor (copy-paste from the terminal, then regex, is the best I've found...).

Using such a feature could be as simple as:

python3 -m mypy --keep-aliases my_test_file.py

I hope you'll see the value of this feature.

In any case, thanks for the great work; mypy is very useful and appreciated :)

[AlexWaygood]

Duplicate of #2968