Allow emscripten-releases-tags/aliases to work be used without SDK deps

This change means that aliases such as `latest` and `3.10.2` can be
used to install jsut the emscipten-releases packages and not the full
SDK with all the dependencies.

This allows uses who do not want to install dependencies such as node,
python or java to still use this package.

These packages were already available to install on their own via
their full names such as:

```
$ ./emsdk install releases-upstream-48ce0b44015d0182fc8c27aa9fbc0a4474b55982-64bi
```

Now these base packages can also be install via:

```
$ ./emsdk install latest-base
```

Fixes: #1142

emsdk.py

@@ -2110,11 +2110,17 @@ def load_file_index_list(filename):
 def load_releases_info():
   if not hasattr(load_releases_info, 'cached_info'):
     try:
-      text = open(sdk_path('emscripten-releases-tags.json'), 'r').read()
-      load_releases_info.cached_info = json.loads(text)
+      with open(sdk_path('emscripten-releases-tags.json')) as f:
+        text = f.read()
+      info = json.loads(text)
     except Exception as e:
       print('Error parsing emscripten-releases-tags.json!')
       exit_with_error(str(e))
+    for key, value in dict(info['aliases']).items():
+      info['aliases'][key + '-base'] = value + '-base'
+    for key, value in dict(info['releases']).items():
+      info['releases'][key + '-base'] = value + '-base'
+    load_releases_info.cached_info = info
 
   return load_releases_info.cached_info
 
@@ -2589,20 +2595,25 @@ def expand_sdk_name(name, activating):
   #   sdk-x.y.z-64bit
   # TODO: support short notation for old builds too?
   fullname = name
-  version = fullname.replace('sdk-', '').replace('releases-', '').replace('-64bit', '').replace('tag-', '')
-  sdk = 'sdk-' if not name.startswith('releases-') else ''
+  version = fullname
+  for pattern in ('sdk-', 'releases-', '-64bit', 'tag-', '-base'):
+    version = version.replace(pattern, '')
+  if name.startswith('releases-') or name.endswith('-base'):
+    sdk_prefix = ''
+  else:
+    sdk_prefix = 'sdk-'
   releases_info = load_releases_info()['releases']
   release_hash = get_release_hash(version, releases_info)
   if release_hash:
     # Known release hash
-    full_name = '%sreleases-%s-64bit' % (sdk, release_hash)
+    full_name = '%sreleases-%s-64bit' % (sdk_prefix, release_hash)
     print("Resolving SDK version '%s' to '%s'" % (version, full_name))
     return full_name
 
   if len(version) == 40:
     global extra_release_tag
     extra_release_tag = version
-    return '%sreleases-%s-64bit' % (sdk, version)
+    return '%sreleases-%s-64bit' % (sdk_prefix, version)
 
   return name
 
@@ -2848,7 +2859,8 @@ def installed_sdk_text(name):
     releases_versions = sorted(load_releases_versions(), key=version_key, reverse=True)
     releases_info = load_releases_info()['releases']
     for ver in releases_versions:
-      print('         %s    %s' % (ver, installed_sdk_text('sdk-releases-%s-64bit' % get_release_hash(ver, releases_info))))
+      if not ver.endswith('-base'):
+        print('         %s    %s' % (ver, installed_sdk_text('sdk-releases-%s-64bit' % get_release_hash(ver, releases_info))))
     print()
 
     # Use array to work around the lack of being able to mutate from enclosing

test/test.py

@@ -252,6 +252,12 @@ def test_install_tool(self):
     # Test that its possible to install emscripten as tool instead of SDK
     checked_call_with_output(emsdk + ' install releases-77b065ace39e6ab21446e13f92897f956c80476a', unexpected='Installing SDK')
 
+  def test_install_alias(self):
+    # 3.1.18 is 49d45744895c7d7e28acd94a385d7ee361653b4a
+    run_emsdk('install 3.1.18-base')
+    checked_call_with_output(emsdk + ' install install 3.1.18-base', unexpected='Installing SDK')
+    checked_call_with_output(emsdk + ' install install releases-upstream-49d45744895c7d7e28acd94a385d7ee361653b4a', unexpected='already downloaded, skipping', unexpected='Downloading:')
+
   def test_activate_missing(self):
     run_emsdk('install latest')
     failing_call_with_output(emsdk + ' activate 2.0.1', expected="error: tool is not installed and therefore cannot be activated: 'releases-13e29bd55185e3c12802bc090b4507901856b2ba-64bit'")