Co-authored-by: Nate Finch <nate.finch@gmail.com>

* Detect deps that have already been invoked

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.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

* Fix nil checks and improve error message

The new mg.F function is the root of the nil-pointer error that can happen
when someone accidentally calls mg.Deps(TargetA()). I have fixed the check
for whether a function was passed so that it won't panic and included
the usage for mg.F in the error message.

I then added a similar check, is the target a function, to
Deps/SerialDeps/CxtSerialDeps functions so that we can give a better
error message when it's passed a non-function. The error message
otherwise will reference mg.F which I don't think will help point people
to the real problem, which is how they called mg.Deps/SerialDeps.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

Co-authored-by: Nate Finch <nate.finch@gmail.com>

* Replaced godoc.org URLs with equivalent pkg.go.dev URLs.

* Switched to full package name for github.com/magefile/mage/mg.

Co-authored-by: Aaron Schlesinger <70865+arschles@users.noreply.github.com>

Co-authored-by: Aaron Schlesinger <70865+arschles@users.noreply.github.com>

* Use magefolder if no directory set and it exists.

If no directory was passed by the user as an explicit option and there is a folder named "magefolder" use that.

Workdir is kept as it is likely still "."

* Remove the default . for -d flag

Also correct os.Stat error checking to expect no error

* Add tests and test data for magefolder

* Rename magefolder and accept untagged files

Magefolder was renamed to magefiles
We now accept files that are not tagged too when using a magefiles directory

* Assume tagging when mix tagging is present

When using magefiles directory, if there are mixed tagging files assume tagging is used for mage files

* Update error format to %v

We support building for older go versions so error formatting should use %v

* sort outputs

* Accept mixed tagging in magefiles folder

When mixed tagging is found within a magefiles folder, opt to use all files

* little tweak to only do go list once when using magefiles directory

* Add magefiles directory information to the website

* Add a preference for mage files over directories

Add a temporary preference for mage files over magefiles directories and warn users this is a temporary functionality leading to a change where directory will be preferred.

Co-authored-by: Nate Finch <natefinch@github.com>