fix #13455 ; joinPath(a,b) now honors trailing slashes in b (or a if b = "")

[timotheecour]

• fix joinPath("", "") is "/" ; should be "" #13455 (fixes remaining cases after fix 3 minor bugs in joinPath (see #13455) #13462 was merged)

• fixed behavior for variadic joinPath which was previously buggy as it didn't follow the same logic as joinPath (see some cases from joinPath("", "") is "/" ; should be "" #13455 ); it now simply calls the same routine as in joinPath.

• behavior for joinPath is now more natural, and consistent with D, nodejs, matlab, and more consistent with how other languages honor trailing slash:

joinPath(a,b) now honors trailing slashes in b (or a if b == "")
in the sense that it ends with a trailing slash if b ends with a trailing slash, or if b is empty and a ends in a trailing slash.

see detailed test cases in PR.

changed behaviors since #13462 :

  ## changed behaviors
  doAssert joinPath("foo", "bar") == "foo/bar" # same as before
  doAssert joinPath("foo", "bar/") == "foo/bar/" # was: foo/bar
  doAssert joinPath("foo", "./") == "foo/" # was: foo
  doAssert joinPath("foo", "", "bar/") == "foo/bar/" # was: foo/bar
  doAssert joinPath("./", ".", "/.") == "." # was: ./ (bug with variadic joinPath)
  doAssert joinPath("", "lib/") == "lib/" # was: lib
  doAssert joinPath("foo", "") == "foo" # was: foo/

it's 100% consistent with how nodejs does it:

Welcome to Node.js v13.8.0.
Type ".help" for more information.
> path.join('foo', '')
'foo'
> path.join('foo', './')
'foo/'
> path.join('foo', '','bar/')
'foo/bar/'
> path.join('./','.','/.')
'.'
> path.join('usr','')
'usr'
> path.join('','lib/')
'lib/'

it's 100% consistent with how matlab, octave does it:

octave:2> fullfile('foo','bar')
ans = foo/bar
octave:3> fullfile('foo','bar/')
ans = foo/bar/
octave:4> fullfile('foo','')
ans = foo
octave:5> fullfile('foo/','')
ans = foo/
octave:6> fullfile('foo/','/bar')
ans = foo/bar

it's also consistent with how D does it

(as far as honoring trailing slash)
(but D differs in some other aspect, buildPath("foo", "/bar") would return "/bar" ; that's a separate topic)

rdmd --eval 'writeln(buildPath("baz/", ""))'
baz/
rdmd --eval 'writeln(buildPath("baz", ""))'
baz
rdmd --eval 'writeln(buildPath("baz", "baz"))'
baz/baz
rdmd --eval 'writeln(buildPath("baz", "baz/"))'
baz/baz/

python

it's now consistent with how python honors trailing slash on tail:
os.path.join("foo","bar")
'foo/bar'
os.path.join("foo","bar/")
'foo/bar/'
but now differs from python in behavior when tail is empty:
os.path.join("foo","")
'foo/'

note

the only controversial aspect in this PR is the edge case of whether
joinPath("foo", "") should be "foo" (as in nodejs, D, go, matlab, and this PR) or "foo/" (as in python).
IMO the way I did it makes more sense, with the semantic of "" being an invalid path (unlike "."), it collapses out, so that
joinPath(foo1, "", foo2, "", "", bar, "") reduces to joinPath(foo1, foo2, bar) for example; it's also more consistent with joinPath("", "") being ""

[Araq]

On Windows tests/stdlib/tos.nim fails with this PR. AFAICT.

[timotheecour]

PTAL, fixed