Replace update_bazel_workspace.sh with a python script (#1271)

This script is (IMO) more readable, but the real reason for this change is that
it raises an error message when the binary package fails to download. (The shell
script silently generated a bogus hash instead, because the shell's `set -e`
builtin does not affect commands executing inside a $() context.
It seemed just as easy to rewrite the script in Python as to fix that.

This change also updates some outdated filename references.

scripts/update_bazel_workspace.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+# This script will update emsdk/bazel/revisons.bzl to the latest version of
+# emscripten. It reads emsdk/emscripten-releases-tags.json to get the latest
+# version number. Then, it downloads the prebuilts for that version and computes
+# the sha256sum for the archive. It then puts all this information into the
+# emsdk/bazel/revisions.bzl file.
+
+import hashlib
+import json
+import os
+import requests
+import sys
+
+STORAGE_URL = 'https://storage.googleapis.com/webassembly/emscripten-releases-builds'
+
+EMSDK_ROOT = os.path.dirname(os.path.dirname(__file__))
+RELEASES_TAGS_FILE = EMSDK_ROOT + '/emscripten-releases-tags.json'
+BAZEL_REVISIONS_FILE = EMSDK_ROOT + '/bazel/revisions.bzl'
+
+
+def get_latest_info():
+    with open(RELEASES_TAGS_FILE) as f:
+        info = json.load(f)
+    latest = info['aliases']['latest']
+    return latest, info['releases'][latest]
+
+
+def get_sha(platform, archive_fmt, latest_hash, arch_suffix=''):
+    r = requests.get(f'{STORAGE_URL}/{platform}/{latest_hash}/wasm-binaries{arch_suffix}.{archive_fmt}')
+    r.raise_for_status()
+    print(f'Fetching {r.url}')
+    h = hashlib.new('sha256')
+    for chunk in r.iter_content(chunk_size=1024):
+        h.update(chunk)
+    return h.hexdigest()
+
+
+def revisions_item(version, latest_hash):
+    return f'''\
+    "{version}": struct(
+        hash = "{latest_hash}",
+        sha_linux = "{get_sha('linux', 'tbz2', latest_hash)}",
+        sha_mac = "{get_sha('mac', 'tbz2', latest_hash)}",
+        sha_mac_arm64 = "{get_sha('mac', 'tbz2', latest_hash, '-arm64')}",
+        sha_win = "{get_sha('win', 'zip', latest_hash)}",
+    ),
+'''
+
+
+def insert_revision(item):
+    with open(BAZEL_REVISIONS_FILE, 'r') as f:
+        lines = f.readlines()
+
+    lines.insert(lines.index('EMSCRIPTEN_TAGS = {\n') + 1, item)
+
+    with open(BAZEL_REVISIONS_FILE, 'w') as f:
+        f.write(''.join(lines))
+
+
+def main(argv):
+    version, latest_hash = get_latest_info()
+    item = revisions_item(version, latest_hash)
+    print('inserting item:')
+    print(item)
+    insert_revision(item)
+
+
+if __name__ == '__main__':
+    sys.exit(main(sys.argv))