Pkg: explain why rm is noop when attempting to remove a package that is dependency

[ihnorton]

This adds an explanation for the no-op message in that situation.
@StefanKarpinski

[StefanKarpinski]

So after trying this out, I realize that this doesn't at all do what you want it too. If r_ == r then this change literally didn't alter the REQUIRE file at all, in which case it's not at all mysterious that it doesn't do anything. Unfortunately, getting the actual warning you want is not entirely straightforward.

[ihnorton]

The message is shown in this situation:

julia> Pkg.add("Winston")
INFO: Installing Winston v0.3.1
INFO: Installing Cairo v0.2.6
INFO: Installing Color v0.2.5
INFO: Installing IniFile v0.2.1
INFO: Installing Tk v0.2.4
INFO: Running build script for package Cairo
INFO: Running build script for package Tk
INFO: REQUIRE updated.

julia> Pkg.rm("Cairo")
INFO: Nothing to be done. Cannot remove yet: Cairo is required by installed package.

but only on the second run when the dependency is installed first:

julia> Pkg.rm("Winston")
INFO: Removing Winston v0.3.1
INFO: Removing Cairo v0.2.6
INFO: Removing IniFile v0.2.1
INFO: Removing Tk v0.2.4
INFO: Removing Color v0.2.5
INFO: REQUIRE updated.

julia> Pkg.add("Cairo")
INFO: Installing Cairo v0.2.6
INFO: Installing Color v0.2.5
INFO: Running build script for package Cairo
INFO: REQUIRE updated.

julia> Pkg.add("Winston")
INFO: Installing Winston v0.3.1
INFO: Installing IniFile v0.2.1
INFO: Installing Tk v0.2.4
INFO: Running build script for package Tk
INFO: REQUIRE updated.

julia> Pkg.rm("Cairo")
INFO: No packages to install, update or remove.
INFO: REQUIRE updated.

julia> Pkg.rm("Cairo")
INFO: Nothing to be done. Cannot remove yet: Cairo is required by installed package.

[StefanKarpinski]

Hmm. I'm not sure how I feel about this only-on-the-second-time behavior. That seems a bit weird.

[tkelman]

Looks like this message was lost somewhere in a pkg1 -> pkg2 rewrite? Ref #7757

Nevermind, it was straight-up reverted in c07c491.