Fix parsing of OPAMFETCH (support quotes / proper POSIX shell syntax) #3991

Job	Run time
Analyse	4s
Cygwin	4s
Build-macOS (4.14.2)	3m 53s
Build-Linux (4.08.1)	3m 30s
Hygiene	36s
Build-macOS (5.2.0)	2m 43s
Build-Linux (4.09.1)	3m 32s
Test-macOS (4.14.2)	24m 15s
Build-Linux (4.10.2)	3m 25s
Test-macOS (5.2.0)	24m 13s
Build-Linux (4.11.2)	3m 39s
Build-Linux (4.12.1)	3m 41s
Build-Linux (4.13.1)	3m 35s
Build-Linux (5.0.0)	3m 41s
Build-Linux (5.1.1)	3m 40s
Build-Linux (4.14.2)	3m 41s
Build-Linux (5.2.0)	3m 37s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	36m 38s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 55s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	39m 2s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	44m 4s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	41m 51s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.2.0)	38m 29s
Solvers-Linux	0s
Doc-Linux	0s
Test-Linux	0s
Cold-Linux	0s
Upgrade-Linux	0s
Solvers-macOS (z3, 4.14.2)	14m 32s
Upgrade-macOS (4.14.2)	1m 58s
Solvers-macOS (z3, 5.2.0)	16m 35s
Upgrade-macOS (5.2.0)	2m 0s
Solvers-macOS (0install, 4.14.2)	1m 21s
Solvers-macOS (0install, 5.2.0)	1m 15s
	6h 11m 29s

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