feat(diagnostic): classify ALE items by their source

[yous]

With coc-python extension, pylint and flake8 enabled and displayByAle is on, say we have this file:

def foo():
    print('foo')

def bar():
    print('bar')


var = baz

Then there are error messages:

test.py|4 col 1 error| [python] E302: expected 2 blank lines, found 1
test.py|8 col 7 error| [python] F821: undefined name 'baz'
test.py|8 col 7 error| [python] E0602: Undefined variable 'baz' (undefined-variable)

Note that they all have [python] header. This patch changes this to:

test.py|4 col 1 error| [flake8] E302: expected 2 blank lines, found 1
test.py|8 col 7 error| [flake8] F821: undefined name 'baz'
test.py|8 col 7 error| [pylint] E0602: Undefined variable 'baz' (undefined-variable)

And this behavior is more similar to ALE's one.

[yous]

Ah, it seems that ale#other_source#ShowResults should be called for each source even when it's empty, according to #1296.

[chemzqm]

The problem is you wouldn't have source name when such kind of diagnostic item doesn't exists any more.

[yous]

Is there a way to get available/enabled sources for each collection?

[chemzqm]

coc-python could add source name to diagnostic message.
Or make your ale support add diagnostics with different source names.