This change plumbs import assertions from SourceTextModuleDescriptor's
ModuleRequestMap into SourceTextModuleInfo via a new ModuleRequest
type, where previously there had been only the specifier.

SourceTextModuleDescriptor::module_map now deduplicates module requests
using the specifier and the import assertions.  Continuing to use the
specifier alone would cause a loss of information in the event that
a module imports from the same specifier multiple times using different
sets of assertions.  Failing to deduplicate at all would result in
multiple requests for statements like `import {a,b,c} from "foo.js"`,
which would be a potential performance issue.  See design doc at
https://docs.google.com/document/d/1yuXgNHSbTAPubT1Mg0JXp5uTrfirkvO1g5cHHCe-LmY
for more detail on this decision.

v8::internal::ModuleRequest holds the assertions as an array of the form
[key1, value1, position1, key2, value2, assertion2, ...].  However the
parser still needs to use a map, since duplicate assertion keys need to
be detected at parse time.  A follow-up  change will ensure that
assertions are sorted using a proper lexicographic sort.

Bug: v8:10958
Change-Id: Iff13fb9a37d58fc1622cd3cce78925ad2b7a14bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2493060
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71066}