Resolves error in prior logic (#102)

Overlay imports don't accurately describe the graph name they're modifying _in_ the graph,
so when ever we have bystanders, we need to use the parsing of the URL name for the relevant
module being extended. This PR updates the prior logic and fixes the test that incorrectly
asserting that [email protected] were symbols that extended the "A" module, when the extend the
"B" module.

Author: heckj

Sources/SymbolKit/UnifiedSymbolGraph/GraphCollector.swift

@@ -139,7 +139,7 @@ extension GraphCollector {
         let isMainSymbolGraph = !url.lastPathComponent.contains("@") && !graph.module.isVirtual
 
         let moduleName: String
-        if isMainSymbolGraph || graph.module.bystanders != nil {
+        if isMainSymbolGraph && graph.module.bystanders == nil {
             // When bystander modules are present, the symbol graph is a cross-import overlay, and
             // we need to preserve the original module name to properly render it. It is still
             // kept with the extension symbols, due to the merging behavior of UnifiedSymbolGraph.
@@ -165,6 +165,9 @@ extension GraphCollector {
             // of the module that was extended (rdar://63200368).
             let fileName = url.lastPathComponent.components(separatedBy: ".symbols.json")[0]
 
+            // Overlay imports of the form `[email protected]` have the graph name being "A", but are symbols
+            // that apply to the "B" module, which isn't included in the graph - so we have to drop back to parsing
+            // the module name from the name of the symbolgraph.
             let fileNameComponents = fileName.components(separatedBy: "@")
             if fileNameComponents.count > 2 {
                 // For a while, cross-import overlay symbol graphs had more than two components:

Tests/SymbolKitTests/UnifiedGraph/GraphCollectorTests.swift

@@ -185,6 +185,6 @@ class GraphCollectorTests: XCTestCase {
 
         let (extendedB, extendedBIsMain) = GraphCollector.moduleNameFor(a_At_B, at: .init(fileURLWithPath: "[email protected]"))
         XCTAssertFalse(extendedBIsMain)
-        XCTAssertEqual("A", extendedB)
+        XCTAssertEqual("B", extendedB)
     }
 }