[lsp] Add Swift / SourceKit-LSP support (#2420)

* [linter] Add LSP support for Swift via sourcekit-lsp

ale_linters/swift/sourcekitlsp.vim

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+" Author: Dan Loman <https://github.com/namolnad>
+" Description: Support for sourcekit-lsp https://github.com/apple/sourcekit-lsp
+
+call ale#Set('sourcekit_lsp_executable', 'sourcekit-lsp')
+
+call ale#linter#Define('swift', {
+\   'name': 'sourcekitlsp',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'sourcekit_lsp_executable')},
+\   'command': '%e',
+\   'project_root': function('ale#swift#FindProjectRoot'),
+\   'language': 'swift',
+\})

autoload/ale/swift.vim

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+" Author: Dan Loman <https://github.com/namolnad>
+" Description: Functions for integrating with Swift tools
+
+" Find the nearest dir containing a Package.swift file and assume it is the root of the Swift project.
+function! ale#swift#FindProjectRoot(buffer) abort
+    let l:swift_config = ale#path#FindNearestFile(a:buffer, 'Package.swift')
+
+    if !empty(l:swift_config)
+        return fnamemodify(l:swift_config, ':h')
+    endif
+
+    return ''
+endfunction

test/command_callback/test_swift_sourcekitlsp_command_callbacks.vader

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+Before:
+  call ale#assert#SetUpLinterTest('swift', 'sourcekitlsp')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  AssertLinter 'sourcekit-lsp', ale#Escape('sourcekit-lsp')
+
+Execute(Should let users configure a global executable and override local paths):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  let g:ale_sourcekit_lsp_executable = '/path/to/custom/sourcekitlsp'
+
+  AssertLinter '/path/to/custom/sourcekitlsp',
+  \ ale#Escape('/path/to/custom/sourcekitlsp')
+
+Execute(The language should be correct):
+  AssertLSPLanguage 'swift'

test/test_swift_find_project_root.vader

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+Before:
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  call ale#test#RestoreDirectory()
+
+Execute(Detect root of Swift project with Package.swift correctly):
+  call ale#test#SetFilename('swift-test-files/swift-package-project/src/folder/dummy.swift')
+  AssertEqual
+  \ ale#path#Simplify(g:dir . '/swift-test-files/swift-package-project'),
+  \ ale#swift#FindProjectRoot(bufnr(''))
+
+Execute(Detect no root in case of non-Package.swift project):
+  call ale#test#SetFilename('swift-test-files/non-swift-package-project/src/folder/dummy.swift')
+  AssertEqual
+  \ '',
+  \ ale#swift#FindProjectRoot(bufnr(''))
+