Correctly handle XML file path

[olblak]

This pullrequest correctly handle XML file path according the different scenarios, relative path, path from scm, etc.

Fix #1210

Test

To test this pull request, you can run the following commands:

go build -o bin/updatecli . 
./bin/updatecli apply --config e2e/updatecli.d/success.d/xml.yaml --clean=true --push=false
cd pkg/plugins/resources/xml
go test .

Additional Information

Tradeoff

Potential improvement

• The source function received a workdir set to the current directory by default. This means we need additional code for each plugin to check the value of that workdir param. This behavior is confusing and should be handled in the "core" codebase