Tools: Correct win_to_unix path handling

[theotherjimmy]

Description

After #7183, many problems cropped up WRT exporters handling of include
paths. This was related to my naive use of os.sep. After calling
win_to_unix, the seperator for the resources object is forced to /.
This PR makes that behaivor explicit by keeping the seperator around.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2677
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7608/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2313
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7608/

[mbed-ci]

Test : SUCCESS

Build number : 2415
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7608/2415