Fix building models in subdirs (#1860)

Fixes #1859

Modifies the changes in #1851,
specifically `SmithyBuild::addSources`, to not check subdirectories for
Smithy files. The previous behavior was to just add whatever path was
given to the method, *not* everything in subdirectories. However, the
change didn't recursively search subdirectores, so you could get build
errors due to missing shapes when running the sources plugin.

This change reverts the behavior of `SmithyBuild::addSource` to only
add whichever path it was given, but still checks to see if the path
is a valid Smithy model.

smithy-build/src/main/java/software/amazon/smithy/build/SmithyBuild.java

@@ -15,8 +15,6 @@
 
 package software.amazon.smithy.build;
 
-import java.io.IOException;
-import java.io.UncheckedIOException;
 import java.nio.file.FileSystems;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -199,23 +197,10 @@ public SmithyBuild config(SmithyBuildConfig config) {
     //
     // Ignores and logs when an unsupported model file is encountered.
     private void addSource(Path path) {
-        try {
-            if (Files.isDirectory(path)) {
-                // Pre-emptively crawl the given models to filter them up front into a flat, file-only, list.
-                Files.list(path).filter(Files::isRegularFile).forEach(this::addSourceFile);
-            } else {
-                addSourceFile(path);
-            }
-        } catch (IOException e) {
-            throw new UncheckedIOException(e);
-        }
-    }
-
-    private void addSourceFile(Path file) {
-        if (!VALID_MODELS.matches(file.getFileName())) {
-            LOGGER.warning("Omitting unsupported Smithy model file from model sources: " + file);
+        if (Files.isRegularFile(path) && !VALID_MODELS.matches(path.getFileName())) {
+            LOGGER.warning("Omitting unsupported Smithy model file from model sources: " + path);
         } else {
-            sources.add(file.toAbsolutePath());
+            sources.add(path.toAbsolutePath());
         }
     }