consolidate_go_stacktrace.py: Add support for replacing Cilium source directory inline

[joestringer]

Add an option to allow the user to specify a cilium source directory,
and substitute all stack traces to Cilium paths with this path.

This way, when using an IDE that allows Ctrl+click navigation in order
to browse to the location, it is easier to just run this script with the
extra dir from the cilium source directory and directly click on the
paths into the codebase for easy stack trace browsing:

$ contrib/scripts/consolidate_go_stacktrace.py stacktrace.txt -s . | head -n 20
114 unique stack traces
18 occurences. Sample stack trace:
sync.runtime_SemacquireMutex(0x30?, 0x0?, 0xc0033a3620?)
        /usr/local/go/src/runtime/sema.go:77 +0x25
sync.(*RWMutex).RLock(...)
        /usr/local/go/src/sync/rwmutex.go:71
github.com/cilium/cilium/pkg/endpoint.(*Endpoint).unconditionalRLock(...)
        ./pkg/endpoint/lock.go:53
github.com/cilium/cilium/pkg/endpoint.(*Endpoint).HasLabels(0xc000ee2000?, 0x2807b69?)
        ./pkg/endpoint/endpoint.go:1028 +0x59
github.com/cilium/cilium/daemon/cmd.(*Daemon).getEndpointList.func1(0xc0006f9e60?, 0xc0030ff590?, 0xc0030ff5a0?)
        ./daemon/cmd/endpoint.go:96 +0x71
created by github.com/cilium/cilium/daemon/cmd.(*Daemon).getEndpointList
        ./daemon/cmd/endpoint.go:94 +0x16f
...

This is a nice shorthand instead of having to pass the output to sed something like this:

$ consolidate_go_stacktrace.py /path/to/my/stacktrace | sed 's;/go/src/github.com/cilium/cilium;/my/custom/path;g`

The first couple of commits are just tidying up to allow the above in
the third commit.

[aditighag]

Looks handy! Left a couple of comments.

[joestringer]

I have another idea, autoreplace the go paths with $GOROOT 😈 ... I'll send a separate PR for that.