Use global using directives for the most common namespaces

[meziantou]

The same namespaces were repeated in almost every file of the analyzer, the code fixers and the test projects. They are now declared once per project, as Using items in the Directory.Build.props of the folder, so that all the Roslyn versions get them.

Global namespaces per project

Meziantou.Analyzer (213 files) — the namespaces used by 49% to 90% of the files:

Namespace	Files that had it
Microsoft.CodeAnalysis	192 (90%)
Meziantou.Framework.Roslyn	174 (81%)
Microsoft.CodeAnalysis.Diagnostics	172 (80%)
System.Collections.Immutable	170 (79%)
Microsoft.CodeAnalysis.Operations	115 (53%)
Meziantou.Analyzer.Internals	106 (49%)

Meziantou.Analyzer.CodeFixers (113 files) — same list without Microsoft.CodeAnalysis.Diagnostics, which is unused there, plus:

Namespace	Files that had it
Microsoft.CodeAnalysis.CodeFixes	108 (95%)
System.Composition	106 (93%)
Microsoft.CodeAnalysis.CodeActions	107 (94%)
Microsoft.CodeAnalysis.Editing	98 (86%)
Microsoft.CodeAnalysis.CSharp.Syntax	96 (84%)
Microsoft.CodeAnalysis.CSharp	65 (57%)

Meziantou.Analyzer.Test (201 files):

Namespace	Files that had it
TestHelper	189 (94%)
Meziantou.Analyzer.Rules	187 (93%)
Meziantou.Analyzer.Test.Helpers	84 (41%)

The remaining namespaces were used by less than a third of the files and are left as-is. using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory; (28% of the code fixers) is also left as-is: making the SyntaxFactory members visible everywhere is more likely to change overload resolution than to help.

Notes for the reviewer

• The files shared by Meziantou.Analyzer and Meziantou.Analyzer.CodeFixers (RuleIdentifiers.cs, Internals/*.cs, Rules/*Common.cs, ...) only lost the using directives that are global in both projects, so they keep compiling in the two projects.
• The using directives that duplicate the implicit global usings of the SDK (System, System.Linq, Xunit, ...) are removed as well: 35 of them, in 30 files.
• Only the using directives at the top of the files are removed, so the code snippets of the tests are untouched.
• The declarations are Using items in the Directory.Build.props of each folder rather than a GlobalUsings.cs file, so that they follow the same rule as the rest of the configuration: every Roslyn version of the folder gets them.

Validation

• dotnet build: succeeds for all the Roslyn versions (4.8, 4.14, 5.0, 5.6, 5.9), no new warning.
• dotnet test: 18012 tests pass, 0 failed, 0 skipped.
• dotnet run --project src/DocumentationGenerator: no change to the markdown files.