Fix loading of cDAC in DAC

[elinor-fung]

When loading the cDAC, we were relying on GetClrModuleDirectory in the DAC actually giving us the DAC directory. With #106217, it explicitly stores/uses the CLR module base. When we first try to load the cDAC in the DAC, that has not yet been set. The current intent is to actually load from next to the DAC, so switch the logic to look next to the current module for the cDAC instead of using the CLR module directory.

Also renamed GetCurrentModuleFileName in utilcode to GetCurrentExecutableFileName so that it accurately represents what it does (I tried to use it and was sad).

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