Prevent variable overwriting in nested calls to dependency provider

[jcar87]

We can have nested calls to find_package, which will be handled by conan_provide_dependency, which is a macro.

Any variable that we set in conan_provide_dependency will be visible by nested calls - if nested calls use the same variable names further on, then the values are rewritten.
As it turns out, this is only causing issues currently as reported here: #570 - note that this PR does not fix that issue.

This PR does not change behaviour - just stylistic changes that make things more evident.

In this PR:

• Use lowercase local-scope variable names for values retrieved from global properties (stylistic - but avoids confusion with the global properties and enforces the notion that they are local scope variables)
• unset() some variables right after we're done using them - before any possible nested call to find_package - not that for these variables, everything was working correctly before this PR, but it was coincidence
• Out of abundance of caution, do not reuse the _find_args variable name, but rather use a variable that is suffixed with the package name being requested - and unset() it immediately after use. The behaviour before this PR was still correct (since the variable was not used after the call to find_package)
• Only mutate CMAKE_MODULE_PATH if we need to fall back to CMake default search behaviour - note that we're still affected by Setting CMAKE_MODULE_PATH before first find_package() does not restore value #570 and the variable is overwritten if we have two nested calls that end up in the fallback block. This will be soon addressed in https://github.com/conan-io/cmake-conan/pull/571/files#diff-d0a3014e65e75c3075c33559a7b0cccb2baaac5e2d2beb52c7884f5bc15bda5f

[memsharded]

Looks necessary clean up, good catch