Detect deps that have already been invoked

[carolynvs]

Detect when a dependency was accidentally already invoked

mg.Deps(MyDep())

Normally this causes a nil pointer panic when we invoke the function

Error: runtime error: invalid memory address or nil pointer dereference

Now it will print the following as soon as its detected

Error: A dependency of the current target was defined improperly, with parenthesis. Dependencies should be defined as mg.Deps(MyDep), not mg.Deps(MyDep())

I have applied this change to checkfn so that it applies to serial and parallel deps.

Fixes #332

[arschles]

LGTM. my only comment probably doesn't need to be addressed in this PR (at least IMO)

[sheldonhull]

I ran into this today, thinking it was an issue with my logging.
Once I changed like below it went away, so thanks for this tip.

mg.Deps(
Js{}.Build(),
BuildAll(),
)

mg.Deps(
Js{}.Build,
BuildAll,
)

I'd like to see this merge get moved forward to support better error output.

[carolynvs]

@natefinch Let me know if you'd prefer to see this fixed in another way. I'm happy to change it up if needed.

[natefinch]

I think this is good but I think the error message needs to change a bit. It could be confusing if you pass in just a nil value of some sort.

I think something more like
fmt.Sprintf("mg.Deps expects a function, but got a nil value of type %T", f)
would work in the more general case.

[carolynvs]

Thanks for the feedback! 🙇‍♀️

After merging with master, I took another look and found a better place to prevent the nil-pointer error. It can occur with mg.F and mg.Deps/SerialDeps.

I incorporated your suggestion for the error message returned by mg.Deps. It doesn't assume that a nil must have come from an invoked function (though that's pretty common). Maybe they passed mg.Deps(3) instead and this will handle that better too.

non-function used as a target dependency: <nil>. The mg.Deps, mg.SerialDeps and mg.CtxDeps functions accept function names, such as mg.Deps(TargetA, TargetB)

When someone calls mg.F directly and passes in an invoked function, this is the error message:

non-function passed to mg.F: <nil>. The mg.F function accept function names, such as mg.F(TargetA, "arg1", "arg2")

[natefinch]

LGTM, thanks!