feat: support bare specifiers in mappings

[dsherret]

Closes #357

A mappings key had to be a path or url, so a specifier that only exists in the deno.json's imports couldn't be mapped by the name that appears in the code. Now a bare specifier key is resolved via the config file's import map:

// deno.json
{
  "imports": {
    "my-lib": "https://deno.land/x/my-lib@39.0.0/src/index.ts"
  }
}

await build({
  // ...etc...
  mappings: {
    "my-lib": {
      name: "npm-target-for-my-lib",
      version: "^39.0.0",
    },
  },
});

Details:

• A key is treated as a path when it has a scheme, when it's relative or absolute, or when it exists on disk (so a key like mod.ts keeps resolving to a file). Everything else is a bare specifier, which means package names containing a dot like chart.js work.
• Urls are still used as-is rather than being run through the import map, which keeps existing mappings resolving the way they always have.
• Resolution uses each entry point as the referrer in turn (main entry points then test entry points) with the referrer's resolution mode, so mappings still resolve when the first entry point isn't the relevant workspace member.
• The "indicated to be mapped ... but were not found" error now shows the key that was specified along with what it resolved to.
• Two mappings that resolve to the same specifier is now an error instead of one silently winning.