Move the core comp unit repo to a separate folder

[patrickbkr]

As proposed in this issue comment, move the core compilation unit repository previously installed directly into <Perl6 home>/ to <Perl6 home>/core. This should continue to work as before, but does not attempt to remove any old stale comp unit repository a previous install might have put there. So some cruft might be left behind.

[patrickbkr]

I just noticed, that this branch is based on my reloc-fix branch and thus contains these changes as well. So please do not merge this branch prior to PR#2939.

[ugexe]

I wonder if we could get away with switching the short name from perl to core. Its already poorly named since its parent folder is perl6...

$ perl6 -e 'say CompUnit::RepositoryRegistry.repository-for-name("perl")'
inst#/Users/ugexe/.rakudobrew/moar-blead-master/install/share/perl6

[patrickbkr]

I'm not too familiar with the ins and outs of the repo stuff, but my guess would be the rename would probably work out without any fallout. @nine Do you have an opinion?

[ugexe]

Only notable thing I can think of is:

rakudo/tools/build/install-core-dist.p6

Lines 23 to 26 in 2566db7

	CompUnit::Repository::Installation.new(
	:prefix(@*ARGS[0]),
	:next-repo(CompUnit::RepositoryRegistry.repository-for-name('perl')),
	)

[patrickbkr]

@ugexe, @niner I tried to also rename the repo from perl to core as per the suggestion of ugexe. I'm absolutely unsure about the changes I did for the rename (I know little about comp repos). It's possible I changed a 'perl' to 'core' that's unrelated to the repo name. Please do review.
Because the release is nearing it's probably a good idea to merge this as soon as possible, so we at least have some time left before the release.

[patrickbkr]

@ugexe, @niner I'd really like to have some kind of approval of this PR before merging, as I don't have much of an opinion on this change. Shall I merge?

[patrickbkr]

I'm merging this now to at least have a little time before the release. If someone has objections, just revert.