Add --incompatible_compact_repo_mapping

[fmeum]

With the flag enabled, <binary>.repo_mapping contains

+deps+*,aaa,_main
+deps+*,dep,+deps+dep1
+deps+*,dep1,+deps+dep1
+deps+*,dep2,+deps+dep2
+deps+*,dep3,+deps+dep3

instead of

+deps+dep1,aaa,_main
+deps+dep1,dep,+deps+dep1
+deps+dep1,dep1,+deps+dep1
+deps+dep1,dep2,+deps+dep2
+deps+dep1,dep3,+deps+dep3
+deps+dep2,aaa,_main
+deps+dep2,dep,+deps+dep1
+deps+dep2,dep1,+deps+dep1
+deps+dep2,dep2,+deps+dep2
+deps+dep2,dep3,+deps+dep3
...

for the deps module extension.

Runfiles libraries have to be updated to find entries using the new format.

Work towards #24808

[fmeum]

@Wyverald Should we ask runfiles libraries to perform a linear match on all lines with prefixes? That's O(num extensions), but avoids specifying the separator char or scheme.