devenv: Escape spaces when relocating pkg-config prefixes - #10589

What

relocate_pkgconfig() writes the real, absolute dependency path into
the downloaded .pc files with a plain byte substitution. pkg-config
tokenizes Cflags/Libs on unescaped whitespace, so when the checkout
lives under a path containing a space (e.g. ~/Desktop/My Projects/kitty)
the -I/-L flags it emits get truncated at the first space and the
build fails, typically with:

kitty/state.h:16:10: fatal error: 'hb.h' file not found

even though harfbuzz's headers are right there on disk.

Fix

Backslash-escape spaces (and literal backslashes) in the new prefix
before substituting it into a .pc file, so pkg-config round-trips the
path correctly. The same function is also used to relocate the prefix
baked into _sysconfigdata_*.py, which is a plain Python string literal
rather than something pkg-config tokenizes, so that case is deliberately
left unescaped.

Testing

Added bypy/devenv_test.go, which fails on master and passes with this
change: it asserts a space in the new prefix is escaped for .pc files
and left untouched for _sysconfigdata_*.py.

I also reproduced this end to end: with a checkout path containing a
space, ./dev.sh build failed with the hb.h error above on master,
and succeeded after this change (all 89 C files compiled and linked).
The new regression test (bypy/devenv_test.go) passes; gofmt and
go vet are clean.

Likely the cause of #8939: the reporter's checkout was under
"3. Ressources" and the -I flags in that log are truncated at the
space.