Add documentation for GHC-21926 (#514)

* Document GHC-21926

* Apply suggested title change

* Explain alphanumeric characters

* Reword the explanation of GHC-21926

* Add full stops and extension to GHC-21926

* Add suggested nuance

Author: FPtje

message-index/messages/GHC-21926/index.md

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+---
+title: Version number or non-alphanumeric character in package name
+summary: Package imports may only consist of alphanumeric characters, and must omit the version.
+severity: error
+introduced: 9.6.1
+extension: PackageImports
+---
+
+Version number or non-alphanumeric character in the package. Each of dash-separated components of package name must consist of alphanumeric characters (as defined by `Data.Char.isAlphaNum`), at least one of which is not a digit.
+Note that you will also see this error if the package name consists only of digits.
+The package name in this case refers to the one mentioned in an import when using the `PackageImports` language extension.

message-index/messages/GHC-21926/non-alphanumeric-in-package/after/Non-alphanumeric-in-package.hs

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+{-# LANGUAGE PackageImports #-}
+
+module NonAlphanumericCharacterNameInPackage where
+
+import "some-existing-package" Package.With.Non.Alphanumeric.Characters

message-index/messages/GHC-21926/non-alphanumeric-in-package/before/Non-alphanumeric-in-package.hs

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+{-# LANGUAGE PackageImports #-}
+
+module NonAlphanumericCharacterNameInPackage where
+
+import "&*$^%!@()" Package.With.Non.Alphanumeric.Characters

message-index/messages/GHC-21926/non-alphanumeric-in-package/index.md

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+---
+title: Non-alphanumeric characters in the package name
+---

message-index/messages/GHC-21926/version-number-in-package/after/Version-name-in-package.hs

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+{-# LANGUAGE PackageImports #-}
+
+module VersionNameInPackage where
+
+import "some-package" Package.With.Version

message-index/messages/GHC-21926/version-number-in-package/before/Version-name-in-package.hs

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+{-# LANGUAGE PackageImports #-}
+
+module VersionNameInPackage where
+
+import "some-package-0.1.2.3" Package.With.Version

message-index/messages/GHC-21926/version-number-in-package/index.md

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+---
+title: Version number in package name
+---