How to create independent 'modules' within the same repository?

[NullPointer]

I am new to bamboo and currently struggling how to achieve the following whilst adhering to best practices (no workarounds or nasty tricks). 

My git repository:

Technology
        |- SharedStuff
        |- Project A
        |- Project B
        |- Project C

I currently have:
- 1 single repository (Technology). 
- 1 Project (Technology)
       |- Plan for A
       |- Plan for B
       |- Plan for C

Actual behaviour:
When pushing a change to Project A, all plans trigger. 

Wanted behaviour:

When pushing a change to Project A, only plan A triggers. 

Known workaround:
Create multiple linked repositories (one per project and use subdirectory expression). If you have 40 projects then you should do this 40 times. 

How does the competition do it?
I come from different tool background where you simply install a maven/ivy plugin and the tool would 'modularise' the projects and build only the modules that have changed (2 min). 
As long as new modules contain the module descriptor (pom or ivy) then new module would be picked up automatically. Is there anything similar in this case?  

Question:
How can I build only changed modules rather than every single plan? 

[NullPointer]

Wow - no answer no one interested on such trivial topic. Questioning the move....