Improve emscripten-releases-tags.txt to support mutliple aliases (#837)

Also, improve reporting of version resolution. e.g.:

```
$ ./emsdk  install sdk-latest
Resolving SDK alias 'latest' to '2.0.23'
Resolving SDK version '2.0.23' to 'sdk-releases-upstream-77b065ace39e6ab21446e13f92897f956c80476a-64bit'
Installing SDK 'sdk-releases-upstream-77b065ace39e6ab21446e13f92897f956c80476a-64bit'..
...
```

emscripten-releases-tags.txt

@@ -1,5 +1,13 @@
 {
+  "aliases": {
     "latest": "2.0.23",
+    "latest-sdk": "latest",
+    "latest-64bit": "latest",
+    "sdk-latest-64bit": "latest",
+    "latest-upstream": "latest",
+    "latest-clang-upstream": "latest",
+    "latest-releases-upstream": "latest"
+  },
   "releases": {
     "2.0.23": "77b065ace39e6ab21446e13f92897f956c80476a",
     "2.0.23-lto": "3f6dbb899f61fab52e4574beb4f7c8382658aa20",

emsdk.py

@@ -2166,18 +2166,27 @@ def is_os_64bit():
   return platform.machine().endswith('64')
 
 
-def find_latest_releases_version():
+def find_latest_version():
+  return resolve_sdk_aliases('latest')
+
+
+def find_latest_hash():
+  version = find_latest_version()
   releases_info = load_releases_info()
-  return releases_info['latest']
+  return releases_info['releases'][version]
 
 
-def find_latest_releases_hash():
+def resolve_sdk_aliases(name, verbose=False):
   releases_info = load_releases_info()
-  return releases_info['releases'][find_latest_releases_version()]
+  while name in releases_info['aliases']:
+    if verbose:
+      print("Resolving SDK alias '%s' to '%s'" % (name, releases_info['aliases'][name]))
+    name = releases_info['aliases'][name]
+  return name
 
 
-def find_latest_releases_sdk(which):
+def find_latest_sdk(which):
-  return 'sdk-releases-%s-%s-64bit' % (which, find_latest_releases_hash())
+  return 'sdk-releases-%s-%s-64bit' % (which, find_latest_hash())
 
 
 def find_tot_sdk():
@@ -2775,12 +2784,7 @@ def expand_sdk_name(name, activating):
     name = name.replace('upstream-master', 'upstream-main')
   if name in ('latest-fastcomp', 'latest-releases-fastcomp', 'tot-fastcomp', 'sdk-nightly-latest'):
     exit_with_fastcomp_error()
-  if name in ('latest', 'sdk-latest', 'latest-64bit', 'sdk-latest-64bit'):
+  if name in ('tot', 'sdk-tot', 'tot-upstream'):
-    # This is effectly the default SDK
-    return str(find_latest_releases_sdk('upstream'))
-  elif name in ('latest-upstream', 'latest-clang-upstream', 'latest-releases-upstream'):
-    return str(find_latest_releases_sdk('upstream'))
-  elif name in ('tot', 'sdk-tot', 'tot-upstream'):
     if activating:
       # When we are activating a tot release, assume that the currently
       # installed SDK, if any, is the tot release we want to activate.
@@ -2791,7 +2795,9 @@ def expand_sdk_name(name, activating):
         debug_print('activating currently installed SDK; not updating tot version')
         return 'sdk-releases-upstream-%s-64bit' % installed
     return str(find_tot_sdk())
-  else:
+
+  name = resolve_sdk_aliases(name, verbose=True)
+
   # check if it's a release handled by an emscripten-releases version,
   # and if so use that by using the right hash. we support a few notations,
   #   x.y.z[-(upstream|fastcomp_])
@@ -2817,13 +2823,17 @@ def expand_sdk_name(name, activating):
         backend = 'upstream'
       else:
         backend = 'fastcomp'
-      return 'sdk-releases-%s-%s-64bit' % (backend, release_hash)
+    full_name = 'sdk-releases-%s-%s-64bit' % (backend, release_hash)
-    elif len(version) == 40:
+    print("Resolving SDK version '%s' to '%s'" % (version, full_name))
+    return full_name
+
+  if len(version) == 40:
     if backend is None:
       backend = 'upstream'
     global extra_release_tag
     extra_release_tag = version
     return 'sdk-releases-%s-%s-64bit' % (backend, version)
+
   return name
 
 
@@ -3043,15 +3053,15 @@ def main(args):
       return 'INSTALLED' if sdk and sdk.is_installed() else ''
 
     if (LINUX or MACOS or WINDOWS) and (ARCH == 'x86' or ARCH == 'x86_64'):
-      print('The *recommended* precompiled SDK download is %s (%s).' % (find_latest_releases_version(), find_latest_releases_hash()))
+      print('The *recommended* precompiled SDK download is %s (%s).' % (find_latest_version(), find_latest_hash()))
       print()
       print('To install/activate it, use one of:')
       print('         latest                  [default (llvm) backend]')
       print('         latest-fastcomp         [legacy (fastcomp) backend]')
       print('')
       print('Those are equivalent to installing/activating the following:')
-      print('         %s             %s' % (find_latest_releases_version(), installed_sdk_text(find_latest_releases_sdk('upstream'))))
+      print('         %s             %s' % (find_latest_version(), installed_sdk_text(find_latest_sdk('upstream'))))
-      print('         %s-fastcomp    %s' % (find_latest_releases_version(), installed_sdk_text(find_latest_releases_sdk('fastcomp'))))
+      print('         %s-fastcomp    %s' % (find_latest_version(), installed_sdk_text(find_latest_sdk('fastcomp'))))
       print('')
     else:
       print('Warning: your platform does not have precompiled SDKs available.')

test/test.py

@@ -140,7 +140,7 @@ int main() {
   def test_list(self):
     # Test we report installed tools properly. The latest version should be
     # installed, but not some random old one.
-    checked_call_with_output(emsdk + ' list', expected=TAGS['latest'] + '    INSTALLED', unexpected='1.39.15    INSTALLED:')
+    checked_call_with_output(emsdk + ' list', expected=TAGS['aliases']['latest'] + '    INSTALLED', unexpected='1.39.15    INSTALLED:')
 
   def test_config_contents(self):
     print('test .emscripten contents')