lsp: Adds support for include paths and have {projectUri}/node_modules added by default.

- Factor out FileRepository's path resolving into own public function.
- Fixes sourceUnitNameToUri() path resolving in relation to include paths being used.
- Adding an solAssert().
- adds nother test for include-paths (bad include)
- Fixes a case on Windows there an ill-formed URI was generated.
- Dropping unnecessary if-branch when translating from sourceUnitName to URI.

test/libsolidity/lsp/include-paths/default_include.sol

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+import "my-module/test.sol";
+
+contract MyContract
+{
+    function f(uint a, uint b) public pure returns (uint)
+    {
+        return MyModule.add(a, b);
+    }
+}
+// ----
+// test:

test/libsolidity/lsp/include-paths/file_at_include_path.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+import "rootlib.sol";
+
+contract MyContract
+{
+}
+// ----
+// rootlib:

test/libsolidity/lsp/include-paths/file_not_found_in_searchpath.sol

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+   import "test.sol";
+// ^^^^^^^^^^^^^^^^^^ @IncludeLocation
+
+contract SomeContract
+{
+}
+// ----
+// file_not_found_in_searchpath: @IncludeLocation 6275

test/libsolidity/lsp/node_modules/my-module/test.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+library MyModule
+{
+    function add(uint a, uint b) public pure returns (uint)
+    {
+        return a + b;
+    }
+}

test/libsolidity/lsp/node_modules/rootlib.sol

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+library MyRootLib
+{
+}