Warn 1.x users when a package is not found that it may be due to our deprecation policy

src/Composer/DependencyResolver/SolverProblemsException.php

@@ -43,7 +43,7 @@ class SolverProblemsException extends \RuntimeException
         }
 
         if (strpos($text, 'could not be found') || strpos($text, 'no matching package found')) {
-            $text .= "\nPotential causes:\n - A typo in the package name\n - The package is not available in a stable-enough version according to your minimum-stability setting\n   see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.\n - It's a private package and you forgot to add a custom repository to find it\n\nRead <https://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.";
+            $text .= "\nPotential causes:\n - This package is not installable via Composer 1.x, see <https://blog.packagist.com/deprecating-composer-1-support/>\n - A typo in the package name\n - The package is not available in a stable-enough version according to your minimum-stability setting\n   see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.\n - It's a private package and you forgot to add a custom repository to find it\n\nRead <https://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.";
         }
 
         if ($hasExtensionProblems) {

tests/Composer/Test/DependencyResolver/SolverTest.php

@@ -709,6 +709,7 @@ class SolverTest extends TestCase
             $msg .= "    - Installation request for a -> satisfiable by A[1.0].\n";
             $msg .= "    - A 1.0 requires b >= 2.0 -> no matching package found.\n\n";
             $msg .= "Potential causes:\n";
+            $msg .= " - This package is not installable via Composer 1.x, see <https://blog.packagist.com/deprecating-composer-1-support/>\n";
             $msg .= " - A typo in the package name\n";
             $msg .= " - The package is not available in a stable-enough version according to your minimum-stability setting\n";
             $msg .= "   see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.\n";

tests/Composer/Test/Fixtures/installer/broken-deps-do-not-replace.test

@@ -31,6 +31,7 @@ Your requirements could not be resolved to an installable set of packages.
     - Installation request for b/b 1.* -> satisfiable by b/b[1.0.0].
 
 Potential causes:
+ - This package is not installable via Composer 1.x, see <https://blog.packagist.com/deprecating-composer-1-support/>
  - A typo in the package name
  - The package is not available in a stable-enough version according to your minimum-stability setting
    see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.

tests/Composer/Test/Fixtures/installer/solver-problems.test

@@ -52,6 +52,7 @@ Your requirements could not be resolved to an installable set of packages.
     - requirer/pkg 1.0.0 requires dependency/pkg 1.0.0 -> satisfiable by dependency/pkg[1.0.0] but these conflict with your requirements or minimum-stability.
 
 Potential causes:
+ - This package is not installable via Composer 1.x, see <https://blog.packagist.com/deprecating-composer-1-support/>
  - A typo in the package name
  - The package is not available in a stable-enough version according to your minimum-stability setting
    see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.