The most common reason for the C# autobuilder to fail is because it
cannot determine a single unique .sln or .proj file to build, instead
reporting multiple sln or proj files at the same shortest depth. This
commit changes this to build all such files, rather than reporting an
error.
This commit fixes a few issues that were identified during the last dist upgrade,
and which were introduced/revealed on 836daaf07b.
- Expand environment variables that are passed from `lgtm.yml` to the autobuilder,
for example `solution: $LGTM_SRC/mysolution.sln`.
- Distinguish between when a build rule is applied automatically and when it is applied
manually via `lgtm.yml`.
- Catch `FileNotFoundException`s when parsing project files and solution files.
When determining the target of `msbuild` or `dotnet build`, first look for `.proj`
files, then `.sln` files, and finally `.csproj`/`.vcxproj` files. In all three cases,
choose the project/solution file closest to the root.
