`NameEnvironment.getModulesDeclaringPackage()` iterates all binary locations linearly

[laeubi]

During analysis of two heapdumps shared here the following issue was discovered as a hotspot that can benefit from optimization:

Performance Data

Metric	WITH transitive	WITHOUT transitive	Ratio
ClasspathLocation.getModulesDeclaringPackage() (µs)	7,443,858	1,671,880	4.5×
ClasspathLocation.findClass() (µs)	4,129,889	1,639,802	2.5×

Description

getModulesDeclaringPackage() iterates over all binary locations and source locations when the lookup strategy is Any or Unnamed:

public char[][] getModulesDeclaringPackage(char[][] packageName, char[] moduleName) {
    // ...
    case Any:
    case Unnamed:
        char[][] names = CharOperation.NO_CHAR_CHAR;
        for (ClasspathLocation location : this.binaryLocations) {      // linear scan
            char[][] declaringModules = location.getModulesDeclaringPackage(pkgName, null);
            if (declaringModules != null)
                names = CharOperation.arrayConcat(names, declaringModules);  // array copy each time!
        }
        for (ClasspathLocation location : this.sourceLocations) {      // another linear scan
            // same pattern
        }

With transitive dependencies, binaryLocations is significantly larger, and this method is called for every package reference during compilation. The CharOperation.arrayConcat() call on every match creates a new array each time, leading to O(n²) allocation behavior when many locations declare the same package.

Similarly, findClass() iterates through all binaryLocations linearly until a match is found:

for (ClasspathLocation classpathLocation : relevantLocations) {
    NameEnvironmentAnswer answer = classpathLocation.findClass(...);
    // ...
}

Suggested Fix

1. Index binary locations by package name: Build a Map<String, List<ClasspathLocation>> so that getModulesDeclaringPackage() only checks locations known to contain a given package, rather than iterating all locations.
2. Use ArrayList + toArray() instead of repeated arrayConcat(): The current approach of appending to char[][] via arrayConcat() is O(n²). Collect results in a List first.
3. Cache package-to-module mappings: Since the classpath doesn't change during a build, cache the result of getModulesDeclaringPackage() for each (packageName, moduleName) pair.