[enh] implement UDA for module declaration

This PR introduce UDA for module declaration.
Usage:
```
///test.d
@UDA(42) module test;
struct UDA
{
    int a;
}

pragma(msg, __traits(getAttributes, test)); //prints "tuple(UDA(42))"

///test2.d
import test;
pragma(msg, __traits(getAttributes, test)); //prints "tuple(UDA(42))"
```
In other words, when __traits(getAttributes, ...) is applied to import statement, it returns attributes of corresponding module.

Mulitple UDAs are allowed:
```
@UDA(42) @UDA(24) module test; //OK
```
```
@(UDA(42), UDA(24)) module test; //OK
```
UDAs can be mixed with `deprecated` attribute:
```
@UDA(42) deprecated module test; //OK
```
```
deprecated @UDA(42) module test; //OK
```

>For example, does this PR support following code?

Yes. See test `test/compilable/imports/udamodule2.d`. Struct UDA is defined in `imports.udamodule2a`, used for annotation `imports.udamodule2` and accessed from `udamodule2`.