Add sourcemap normalization to fetch loader

[guybedford]

This fixes up the source maps support when using the fetch loader.

[github-actions]

File size impact

Merging fetch-sourcemaps into master will impact 3 files in 2 groups.

browser (2/2)

File	raw	gzip	brotli	Event
dist/s.min.js	+173 (7,443)	+57 (2,925)	+56 (2,648)	modified
dist/system.min.js	+173 (11,504)	+57 (4,378)	+49 (3,935)	modified
Total size impact	+346 (18,947)	+114 (7,303)	+105 (6,583)

node (1/1)

File	raw	gzip	brotli	Event
dist/system-node.cjs	+441 (201,442)	+93 (52,094)	+102 (43,798)	modified
Total size impact	+441 (201,442)	+93 (52,094)	+102 (43,798)

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#366727316}