Instead of developing plugins against a single, fixed Plugin API version - `"composer-plugin-api": "1.0.0"`, this change will allow plugin developers to use versions like `"composer-plugin-api": "~1.1"` or `"composer-plugin-api": ">=2.1 <3.0"`, aka actual Composer-compatible constraints.
Only the "1.0", "1.0.0" and "1.0.0" Plugin API versions will be regarded as BC versions, and internally converted to "^1.0"; every other declared version string will be kept as it is.
Because of this new constraint flexibility, plugin version mismatches will be skipped, which means those plugin will NOT be registered to the system. Previously, a mismatch triggered a warning, but plugins were still registered.
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